Video or image coding applying adaptive loop filter

ABSTRACT

An image decoding method, according to the present disclosure may comprise: acquiring, from a bitstream, image information including adaptive loop filter (ALF) information including, alternative filter information for a chroma component of a current block, and residual information; generating reconstructed samples for the current block on the basis of the residual information; and generating modified reconstructed samples for the chroma component of the current block on the basis of the alternative filter information.

This is a Continuation of U.S. patent application Ser. No. 17/570,237,filed Jan. 6, 2022, which is a Bypass of PCT Application No.PCT/KR2020/008941, with an international filing date of Jul. 8, 2020,which claims the benefit of U.S. Provisional Application No. 62/871,223,filed on Jul. 8, 2019, all of which are incorporated by reference intheir entirety herein.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to video or image coding applyingadaptive loop filter.

Related Art

Recently, demand for high-resolution, high-quality image/video such as4K or 8K or higher ultra high definition (UHD) image/video has increasedin various fields. As image/video data has high resolution and highquality, the amount of information or bits to be transmitted increasesrelative to the existing image/video data, and thus, transmitting imagedata using a medium such as an existing wired/wireless broadband line oran existing storage medium or storing image/video data using existingstorage medium increase transmission cost and storage cost.

In addition, interest and demand for immersive media such as virtualreality (VR) and artificial reality (AR) content or holograms hasrecently increased and broadcasting for image/video is havingcharacteristics different from reality images such as game images hasincreased.

Accordingly, a highly efficient image/video compression technology isrequired to effectively compress, transmit, store, and reproduceinformation of a high-resolution, high-quality image/video havingvarious characteristics as described above.

In addition, there is a discussion on filtering techniques using theadaptive loop filtering (ALF) to improve compression efficiency andincrease subjective/objective visual quality. In order to efficientlyapply these techniques, there is a need for a method for efficientlysignaling related information.

SUMMARY

According to an embodiment of the present disclosure, an image decodingmethod performed by a decoding apparatus is provided. The methodincludes: obtaining image information including adaptive loop filter(ALF) information and residual information from a bitstream; generatingreconstructed samples for a current block based on the residualinformation; deriving filter coefficients for the ALF based on the ALFinformation; and generating modified reconstructed samples for thecurrent block based on the reconstructed samples and the filtercoefficients, wherein the ALF information includes alternative filterinformation for a chroma component of the current block, and the filtercoefficients for the chroma component of the current block are generatedbased on the alternative filter information.

According to another embodiment of the present disclosure, a videoencoding method performed by an encoding apparatus is provided. Themethod includes: deriving prediction samples for a current block in acurrent picture; deriving residual samples for the current block basedon the prediction samples; generating residual information andreconstructed samples based on the residual samples; deriving filtercoefficients for an adaptive loop filter (ALF) for the reconstructedsamples; generating ALF information based on the filter coefficients;and encoding image information including the ALF information and theresidual information, wherein the ALF information includes alternativefilter information for a chroma component of the current block, and thealternative filter information indicates an alternative filter forderiving modified reconstructed samples for the chroma component of thecurrent block.

According to another embodiment of the present disclosure, there isprovided a computer-readable digital storage medium in which a bitstreamincluding image information causing a decoding apparatus to perform animage decoding method is stored. The image decoding method includes:obtaining image information including adaptive loop filter (ALF)information and residual information from a bitstream; generatingreconstructed samples for a current block based on the residualinformation; deriving filter coefficients for the ALF based on the ALFinformation; and generating modified reconstructed samples for thecurrent block based on the reconstructed samples and the filtercoefficients, wherein the ALF information includes alternative filterinformation on a chroma component of the current block, and the filtercoefficients for the chroma component of the current block are generatedbased on the alternative filter information.

Advantageous Effects

According to an embodiment of the present disclosure, overallimage/video compression efficiency can be increased.

According to an embodiment of the present disclosure,subjective/objective visual quality can be increased through efficientfiltering.

According to an embodiment of the present disclosure, ALF-relatedinformation can be efficiently signaled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an example of a video/image codingsystem to which the embodiments of the present disclosure may beapplied.

FIG. 2 is a diagram schematically illustrating a configuration of avideo/image encoding apparatus to which the embodiments of the presentdisclosure may be applied.

FIG. 3 is a diagram schematically illustrating a configuration of avideo/image decoding apparatus to which the embodiments of the presentdisclosure may be applied.

FIG. 4 shows a block diagram of context-adaptive arithmetic coding(CABAC).

FIG. 5 shows an entropy encoder of an encoding apparatus.

FIG. 6 shows an entropy decoder of a decoding apparatus.

FIG. 7 exemplarily shows a hierarchical structure for a codedimage/video.

FIG. 8 is a flowchart schematically illustrating an example of an ALFprocess.

FIG. 9 shows examples of the shapes of an ALF filters.

FIG. 10 shows an example of a hierarchical structure of ALF data.

FIG. 11 shows another example of a hierarchical structure of ALF data.

FIG. 12 shows fixed orders corresponding to position dependent filtercoefficients.

FIG. 13 and FIG. 14 schematically show an example of a video/imageencoding method and related components according to embodiment(s) of thepresent disclosure.

FIG. 15 and FIG. 16 schematically show an example of an image/videodecoding method and related components according to an embodiment of thepresent disclosure.

FIG. 17 shows an example of a content streaming system to whichembodiments disclosed in the present disclosure may be applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

While the present disclosure can have a variety of modifications andconfigurations, certain embodiments have been illustrated in thedrawings and explained in detail herein. However, this should not beconstrued as limiting the present disclosure to any specific embodiment.The terms used herein are presented for the description of the specificembodiments but are not intended to limit the technical idea of thepresent disclosure. The terms of a singular form may include pluralforms unless otherwise specified. It will be understood that the terms“comprising” or “having,” when used herein, specify the presence ofstated features, integers, steps, operations, elements, components, orcombinations thereof, but do not preclude the presence or addition ofone or more other features, integers, steps, operations, elements,components, or combinations thereof.

Each of the components in the drawings described in the presentdisclosure are illustrated independently for the convenience ofdescription regarding different characteristic functions, and do notmean that the components are implemented in separate hardware orseparate software. For example, two or more of each configuration may becombined to form one configuration, or one configuration may be dividedinto a plurality of configurations. Embodiments in which eachconfiguration is integrated and/or separated are also included in thescope of the present disclosure without departing from the spirit of thepresent disclosure.

In the present specification, “A or B” may mean “only A”, “only B” or“both A and B.” In other words, in the present specification, “A or B”may be interpreted as “A and/or B”. For example, in the presentspecification, “A, B, or C” may mean “only A”, “only B”, “only C”, or“any combination of A, B, C.”

A slash (/) or comma used in the present specification may mean“and/or.” For example, “A/B” may mean “A and/or B.” Accordingly, “A/B”may mean “only A,” “only B”, or “both A and B.” For example, “A, B, C”may mean “A, B, or C.”

In the present specification, “at least one of A and B” may mean “onlyA”, “only B,” or “both A and B.” In addition, in the presentspecification, the expression “at least one of A or B” or “at least oneof A and/or B” may be interpreted as “at least one of A and B”.

In addition, in the present specification, “at least one of A, B, and C”may mean “only A”, “only B”, “only C”, or “any combination of A, B, andC.” In addition, “at least one of A, B, or C” or “at least one of A, B,and/or C” may mean “at least one of A, B, and C.”

In addition, a parenthesis used in the present specification may mean“for example.” Specifically, when indicated as “prediction (intraprediction),” it may mean that “intra prediction” is proposed as anexample of the “prediction.” In other words, the “prediction” of thepresent specification is not limited to “intra prediction,” and “intraprediction” may be proposed as an example of the “prediction.” Inaddition, when indicated as “prediction (i.e., intra prediction),” itmay also mean that “intra prediction” is proposed as an example of the“prediction.”

A technical feature described individually in one figure in the presentspecification may be individually implemented, or may be simultaneouslyimplemented.

Hereinafter, preferred embodiments of the present disclosure will bedescribed in more detail with reference to the accompanying drawings.Hereinafter, the same reference numerals are used for the samecomponents in the drawings, and repeated descriptions of the samecomponents may be omitted.

FIG. 1 schematically illustrates an example of a video/image codingsystem to which the present disclosure may be applied.

Referring to FIG. 1 , a video/image coding system may include a firstdevice (a source device) and a second device (a reception device). Thesource device may transmit encoded video/image information or data tothe reception device through a digital storage medium or network in theform of a file or streaming.

The source device may include a video source, an encoding apparatus, anda transmitter. The receiving device may include a receiver, a decodingapparatus, and a renderer. The encoding apparatus may be called avideo/image encoding apparatus, and the decoding apparatus may be calleda video/image decoding apparatus. The transmitter may be included in theencoding apparatus. The receiver may be included in the decodingapparatus. The renderer may include a display, and the display may beconfigured as a separate device or an external component.

The video source may acquire video/image through a process of capturing,synthesizing, or generating the video/image. The video source mayinclude a video/image capture device and/or a video/image generatingdevice. The video/image capture device may include, for example, one ormore cameras, video/image archives including previously capturedvideo/images, and the like. The video/image generating device mayinclude, for example, computers, tablets and smartphones, and may(electronically) generate video/images. For example, a virtualvideo/image may be generated through a computer or the like. In thiscase, the video/image capturing process may be replaced by a process ofgenerating related data.

The encoding apparatus may encode input video/image. The encodingapparatus may perform a series of procedures such as prediction,transform, and quantization for compaction and coding efficiency. Theencoded data (encoded video/image information) may be output in the formof a bitstream.

The transmitter may transmit the encoded image/image information or dataoutput in the form of a bitstream to the receiver of the receivingdevice through a digital storage medium or a network in the form of afile or streaming. The digital storage medium may include variousstorage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and thelike. The transmitter may include an element for generating a media filethrough a predetermined file format and may include an element fortransmission through a broadcast/communication network. The receiver mayreceive/extract the bitstream and transmit the received bitstream to thedecoding apparatus.

The decoding apparatus may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding apparatus.

The renderer may render the decoded video/image. The renderedvideo/image may be displayed through the display.

The present disclosure relates to video/image coding. For example, amethod/embodiment disclosed in the present document may be applied to amethod disclosed in the versatile video coding (VVC) standard, theessential video coding (EVC) standard, the AOMedia Video 1 (AV1)standard, the 2nd generation of audio video coding standard (AVS2) orthe next generation video/image coding standard (e.g., H.267, H.268, orthe like).

The present document suggests various embodiments of video/image coding,and the above embodiments may also be performed in combination with eachother unless otherwise specified.

In the present document, a video may refer to a series of images overtime. A picture generally refers to the unit representing one image at aparticular time frame, and a slice/tile refers to the unit constitutinga part of the picture in terms of coding. A slice/tile may include oneor more coding tree units (CTUs). One picture may consist of one or moreslices/tiles.

A tile is a rectangular region of CTUs within a particular tile columnand a particular tile row in a picture. The tile column is a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements in the picture parameter set. Thetile row is a rectangular region of CTUs having a height specified bysyntax elements in the picture parameter set and a width equal to thewidth of the picture. A tile scan is a specific sequential ordering ofCTUs partitioning a picture in which the CTUs are ordered consecutivelyin CTU raster scan in a tile whereas tiles in a picture are orderedconsecutively in a raster scan of the tiles of the picture. A slice mayinclude multiple complete tiles or multiple consecutive CTU rows in onetile of a picture, which may be included in one NAL unit. In thisdocument, tile group and slice can be used interchangeably. For example,in this document, a tile group/tile group header may be referred to as aslice/slice header.

Meanwhile, one picture may be divided into two or more subpictures. Asubpicture may be a rectangular region of one or more slices within apicture.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma (ex. cb, cr) blocks. The unit may be used interchangeably withterms such as block or area in some cases. In a general case, an M×Nblock may include samples (or sample arrays) or a set (or array) oftransform coefficients of M columns and N rows. Alternatively, thesample may mean a pixel value in the spatial domain, and when such apixel value is transformed to the frequency domain, it may mean atransform coefficient in the frequency domain.

FIG. 2 is a diagram schematically illustrating a configuration of avideo/image encoding apparatus to which the present disclosure may beapplied. Hereinafter, a video encoding apparatus may include an imageencoding apparatus.

Referring to FIG. 2 , the encoding apparatus 200 includes an imagepartitioner 210, a predictor 220, a residual processor 230, and anentropy encoder 240, an adder 250, a filter 260, and a memory 270. Thepredictor 220 may include an inter predictor 221 and an intra predictor222. The residual processor 230 may include a transformer 232, aquantizer 233, a dequantizer 234, and an inverse transformer 235. Theresidual processor 230 may further include a subtractor 231. The adder250 may be called a reconstructor or a reconstructed block generator.The image partitioner 210, the predictor 220, the residual processor230, the entropy encoder 240, the adder 250, and the filter 260 may beconfigured by at least one hardware component (ex. An encoder chipset orprocessor) according to an embodiment. In addition, the memory 270 mayinclude a decoded picture buffer (DPB) or may be configured by a digitalstorage medium. The hardware component may further include the memory270 as an internal/external component.

The image partitioner 210 may partition an input image (or a picture ora frame) input to the encoding apparatus 200 into one or moreprocessors. For example, the processor may be called a coding unit (CU).In this case, the coding unit may be recursively partitioned accordingto a quad-tree binary-tree ternary-tree (QTBTTT) structure from a codingtree unit (CTU) or a largest coding unit (LCU). For example, one codingunit may be partitioned into a plurality of coding units of a deeperdepth based on a quad tree structure, a binary tree structure, and/or aternary structure. In this case, for example, the quad tree structuremay be applied first and the binary tree structure and/or ternarystructure may be applied later. Alternatively, the binary tree structuremay be applied first. The coding procedure according to the presentdisclosure may be performed based on the final coding unit that is nolonger partitioned. In this case, the largest coding unit may be used asthe final coding unit based on coding efficiency according to imagecharacteristics, or if necessary, the coding unit may be recursivelypartitioned into coding units of deeper depth and a coding unit havingan optimal size may be used as the final coding unit. Here, the codingprocedure may include a procedure of prediction, transform, andreconstruction, which will be described later. As another example, theprocessor may further include a prediction unit (PU) or a transform unit(TU). In this case, the prediction unit and the transform unit may besplit or partitioned from the aforementioned final coding unit. Theprediction unit may be a unit of sample prediction, and the transformunit may be a unit for deriving a transform coefficient and/or a unitfor deriving a residual signal from the transform coefficient.

The unit may be used interchangeably with terms such as block or area insome cases. In a general case, an M×N block may represent a set ofsamples or transform coefficients composed of M columns and N rows. Asample may generally represent a pixel or a value of a pixel, mayrepresent only a pixel/pixel value of a luma component or represent onlya pixel/pixel value of a chroma component. A sample may be used as aterm corresponding to one picture (or image) for a pixel or a pel.

In the encoding apparatus 200, a prediction signal (predicted block,prediction sample array) output from the inter predictor 221 or theintra predictor 222 is subtracted from an input image signal (originalblock, original sample array) to generate a residual signal residualblock, residual sample array), and the generated residual signal istransmitted to the transformer 232. In this case, as shown, a unit forsubtracting a prediction signal (predicted block, prediction samplearray) from the input image signal (original block, original samplearray) in the encoding apparatus 200 may be called a subtractor 231. Thepredictor may perform prediction on a block to be processed(hereinafter, referred to as a current block) and generate a predictedblock including prediction samples for the current block. The predictormay determine whether intra prediction or inter prediction is applied ona current block or CU basis. As described later in the description ofeach prediction mode, the predictor may generate various informationrelated to prediction, such as prediction mode information, and transmitthe generated information to the entropy encoder 240. The information onthe prediction may be encoded in the entropy encoder 240 and output inthe form of a bitstream.

The intra predictor 222 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The non-directional mode may include, for example,a DC mode and a planar mode. The directional mode may include, forexample, 33 directional prediction modes or 65 directional predictionmodes according to the degree of detail of the prediction direction.However, this is merely an example, more or less directional predictionmodes may be used depending on a setting. The intra predictor 222 maydetermine the prediction mode applied to the current block by using aprediction mode applied to a neighboring block.

The inter predictor 221 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. Here, in order to reduce theamount of motion information transmitted in the inter prediction mode,the motion information may be predicted in units of blocks, sub-blocks,or samples based on correlation of motion information between theneighboring block and the current block. The motion information mayinclude a motion vector and a reference picture index. The motioninformation may further include inter prediction direction (L0prediction, L1 prediction, Bi prediction, etc.) information. In the caseof inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. The referencepicture including the reference block and the reference pictureincluding the temporal neighboring block may be the same or different.The temporal neighboring block may be called a collocated referenceblock, a co-located CU (colCU), and the like, and the reference pictureincluding the temporal neighboring block may be called a collocatedpicture (colPic). For example, the inter predictor 221 may configure amotion information candidate list based on neighboring blocks andgenerate information indicating which candidate is used to derive amotion vector and/or a reference picture index of the current block.Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the interpredictor 221 may use motion information of the neighboring block asmotion information of the current block. In the skip mode, unlike themerge mode, the residual signal may not be transmitted. In the case ofthe motion vector prediction (MVP) mode, the motion vector of theneighboring block may be used as a motion vector predictor and themotion vector of the current block may be indicated by signaling amotion vector difference.

The predictor 220 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply both intra prediction and inter prediction.This may be called combined inter and intra prediction (CIIP). Inaddition, the predictor may be based on an intra block copy (IBC)prediction mode or a palette mode for prediction of a block. The IBCprediction mode or palette mode may be used for content image/videocoding of a game or the like, for example, screen content coding (SCC).The IBC basically performs prediction in the current picture but may beperformed similarly to inter prediction in that a reference block isderived in the current picture. That is, the IBC may use at least one ofthe inter prediction techniques described in the present disclosure. Thepalette mode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The prediction signal generated by the predictor (including the interpredictor 221 and/or the intra predictor 222) may be used to generate areconstructed signal or to generate a residual signal. The transformer232 may generate transform coefficients by applying a transformtechnique to the residual signal. For example, the transform techniquemay include at least one of a discrete cosine transform (DCT), adiscrete sine transform (DST), a karhunen-loéve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform generated based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 233 may quantize the transform coefficients and transmitthem to the entropy encoder 240 and the entropy encoder 240 may encodethe quantized signal (information on the quantized transformcoefficients) and output a bitstream. The information on the quantizedtransform coefficients may be referred to as residual information. Thequantizer 233 may rearrange block type quantized transform coefficientsinto a one-dimensional vector form based on a coefficient scanning orderand generate information on the quantized transform coefficients basedon the quantized transform coefficients in the one-dimensional vectorform. Information on transform coefficients may be generated. Theentropy encoder 240 may perform various encoding methods such as, forexample, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 240 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(ex. values of syntax elements, etc.) together or separately. Encodedinformation (ex. encoded video/image information) may be transmitted orstored in units of NALs (network abstraction layer) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. In the presentdisclosure, information and/or syntax elements transmitted/signaled fromthe encoding apparatus to the decoding apparatus may be included invideo/picture information. The video/image information may be encodedthrough the above-described encoding procedure and included in thebitstream. The bitstream may be transmitted over a network or may bestored in a digital storage medium. The network may include abroadcasting network and/or a communication network, and the digitalstorage medium may include various storage media such as USB, SD, CD,DVD, Blu-ray, HDD, SSD, and the like. A transmitter (not shown)transmitting a signal output from the entropy encoder 240 and/or astorage unit (not shown) storing the signal may be included asinternal/external element of the encoding apparatus 200, andalternatively, the transmitter may be included in the entropy encoder240.

The quantized transform coefficients output from the quantizer 233 maybe used to generate a prediction signal. For example, the residualsignal (residual block or residual samples) may be reconstructed byapplying dequantization and inverse transform to the quantized transformcoefficients through the dequantizer 234 and the inverse transformer235. The adder 250 adds the reconstructed residual signal to theprediction signal output from the inter predictor 221 or the intrapredictor 222 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 250 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied duringpicture encoding and/or reconstruction.

The filter 260 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter260 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 270, specifically, a DPB of thememory 270. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 260 may generate variousinformation related to the filtering and transmit the generatedinformation to the entropy encoder 240 as described later in thedescription of each filtering method. The information related to thefiltering may be encoded by the entropy encoder 240 and output in theform of a bitstream.

The modified reconstructed picture transmitted to the memory 270 may beused as the reference picture in the inter predictor 221. When the interprediction is applied through the encoding apparatus, predictionmismatch between the encoding apparatus 200 and the decoding apparatus300 may be avoided and encoding efficiency may be improved.

The DPB of the memory 270 DPB may store the modified reconstructedpicture for use as a reference picture in the inter predictor 221. Thememory 270 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 221 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 270 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra predictor 222.

In the present disclosure, at least one of quantization/dequantizationand/or transform/inverse transform may be omitted. Whenquantization/dequantization is omitted, a quantized transformcoefficient may be referred to as a transform coefficient. Whentransform/inverse transform is omitted, the transform coefficient may bereferred to as a coefficient or a residual coefficient, or may still bereferred to as a transform coefficient for consistency of expression.

Further, in the present disclosure, a quantized transform coefficientand a transform coefficient may be referred to as a transformcoefficient and a scaled transform coefficient, respectively. In thiscase, residual information may include information on a transformcoefficient(s), and the information on the transform coefficient(s) maybe signaled through a residual coding syntax. Transform coefficients maybe derived based on the residual information (or information on thetransform coefficient(s)), and scaled transform coefficients may bederived through inverse transform (scaling) of the transformcoefficients. Residual samples may be derived based on the inversetransform (transform) of the scaled transform coefficients. Thesedetails may also be applied/expressed in other parts of the presentdisclosure.

FIG. 3 is a diagram schematically illustrating a configuration of avideo/image decoding apparatus to which the present disclosure may beapplied.

Referring to FIG. 3 , the decoding apparatus 300 may include an entropydecoder 310, a residual processor 320, a predictor 330, an adder 340, afilter 350, a memory 360. The predictor 330 may include an intrapredictor 331 and an inter predictor 332. The residual processor 320 mayinclude a dequantizer 321 and an inverse transformer 321. The entropydecoder 310, the residual processor 320, the predictor 330, the adder340, and the filter 350 may be configured by a hardware component (ex. Adecoder chipset or a processor) according to an embodiment. In addition,the memory 360 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium. The hardware component mayfurther include the memory 360 as an internal/external component.

When a bitstream including video/image information is input, thedecoding apparatus 300 may reconstruct an image corresponding to aprocess in which the video/image information is processed in theencoding apparatus of FIG. 2 . For example, the decoding apparatus 300may derive units/blocks based on block partition related informationobtained from the bitstream. The decoding apparatus 300 may performdecoding using a processor applied in the encoding apparatus. Thus, theprocessor of decoding may be a coding unit, for example, and the codingunit may be partitioned according to a quad tree structure, binary treestructure and/or ternary tree structure from the coding tree unit or thelargest coding unit. One or more transform units may be derived from thecoding unit. The reconstructed image signal decoded and output throughthe decoding apparatus 300 may be reproduced through a reproducingapparatus.

The decoding apparatus 300 may receive a signal output from the encodingapparatus of FIG. 2 in the form of a bitstream, and the received signalmay be decoded through the entropy decoder 310. For example, the entropydecoder 310 may parse the bitstream to derive information (ex.video/image information) necessary for image reconstruction (or picturereconstruction). The video/image information may further includeinformation on various parameter sets such as an adaptation parameterset (APS), a picture parameter set (PPS), a sequence parameter set(SPS), or a video parameter set (VPS). In addition, the video/imageinformation may further include general constraint information. Thedecoding apparatus may further decode picture based on the informationon the parameter set and/or the general constraint information.Signaled/received information and/or syntax elements described later inthe present disclosure may be decoded may decode the decoding procedureand obtained from the bitstream. For example, the entropy decoder 310decodes the information in the bitstream based on a coding method suchas exponential Golomb coding, CAVLC, or CABAC, and output syntaxelements required for image reconstruction and quantized values oftransform coefficients for residual. More specifically, the CABACentropy decoding method may receive a bin corresponding to each syntaxelement in the bitstream, determine a context model using a decodingtarget syntax element information, decoding information of a decodingtarget block or information of a symbol/bin decoded in a previous stage,and perform an arithmetic decoding on the bin by predicting aprobability of occurrence of a bin according to the determined contextmodel, and generate a symbol corresponding to the value of each syntaxelement. In this case, the CABAC entropy decoding method may update thecontext model by using the information of the decoded symbol/bin for acontext model of a next symbol/bin after determining the context model.The information related to the prediction among the information decodedby the entropy decoder 310 may be provided to the predictor (the interpredictor 332 and the intra predictor 331), and the residual value onwhich the entropy decoding was performed in the entropy decoder 310,that is, the quantized transform coefficients and related parameterinformation, may be input to the residual processor 320. The residualprocessor 320 may derive the residual signal (the residual block, theresidual samples, the residual sample array). In addition, informationon filtering among information decoded by the entropy decoder 310 may beprovided to the filter 350. Meanwhile, a receiver (not shown) forreceiving a signal output from the encoding apparatus may be furtherconfigured as an internal/external element of the decoding apparatus300, or the receiver may be a component of the entropy decoder 310.Meanwhile, the decoding apparatus according to the present disclosuremay be referred to as a video/image/picture decoding apparatus, and thedecoding apparatus may be classified into an information decoder(video/image/picture information decoder) and a sample decoder(video/image/picture sample decoder). The information decoder mayinclude the entropy decoder 310, and the sample decoder may include atleast one of the dequantizer 321, the inverse transformer 322, the adder340, the filter 350, the memory 360, the inter predictor 332, and theintra predictor 331.

The dequantizer 321 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 321 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock form. In this case, the rearrangement may be performed based onthe coefficient scanning order performed in the encoding apparatus. Thedequantizer 321 may perform dequantization on the quantized transformcoefficients by using a quantization parameter (ex. quantization stepsize information) and obtain transform coefficients.

The inverse transformer 322 inversely transforms the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The predictor may perform prediction on the current block and generate apredicted block including prediction samples for the current block. Thepredictor may determine whether intra prediction or inter prediction isapplied to the current block based on the information on the predictionoutput from the entropy decoder 310 and may determine a specificintra/inter prediction mode.

The predictor 330 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply intra prediction and inter prediction. Thismay be called combined inter and intra prediction (CIIP). In addition,the predictor may be based on an intra block copy (IBC) prediction modeor a palette mode for prediction of a block. The IBC prediction mode orpalette mode may be used for content image/video coding of a game or thelike, for example, screen content coding (SCC). The IBC basicallyperforms prediction in the current picture but may be performedsimilarly to inter prediction in that a reference block is derived inthe current picture. That is, the IBC may use at least one of the interprediction techniques described in the present disclosure. The palettemode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The intra predictor 331 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The intra predictor 331 may determine theprediction mode applied to the current block by using a prediction modeapplied to a neighboring block.

The inter predictor 332 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. In this case, in order to reducethe amount of motion information transmitted in the inter predictionmode, motion information may be predicted in units of blocks,sub-blocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter predictor 332 may configure a motion information candidate listbased on neighboring blocks and derive a motion vector of the currentblock and/or a reference picture index based on the received candidateselection information. Inter prediction may be performed based onvarious prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 340 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the predictor (including the interpredictor 332 and/or the intra predictor 331). If there is no residualfor the block to be processed, such as when the skip mode is applied,the predicted block may be used as the reconstructed block.

The adder 340 may be called reconstructor or a reconstructed blockgenerator. The generated reconstructed signal may be used for intraprediction of a next block to be processed in the current picture, maybe output through filtering as described below, or may be used for interprediction of a next picture.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied in thepicture decoding process.

The filter 350 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter350 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 360, specifically, a DPB of thememory 360. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 360may be used as a reference picture in the inter predictor 332. Thememory 360 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 260 so as to be utilized as the motion informationof the spatial neighboring block or the motion information of thetemporal neighboring block. The memory 360 may store reconstructedsamples of reconstructed blocks in the current picture and transfer thereconstructed samples to the intra predictor 331.

In the present document, the embodiments described in the filter 260,the inter predictor 221, and the intra predictor 222 of the encodingapparatus 200 may be the same as or respectively applied to correspondto the filter 350, the inter predictor 332, and the intra predictor 331of the decoding apparatus 300. The same may also apply to the unit 332and the intra predictor 331.

As described above, in video coding, prediction is performed to increasecompression efficiency. Through this, it is possible to generate apredicted block including prediction samples for a current block, whichis a block to be coded. Here, the predicted block includes predictionsamples in a spatial domain (or pixel domain). The predicted block isderived equally from the encoding device and the decoding device, andthe encoding device decodes information (residual information) on theresidual between the original block and the predicted block, not theoriginal sample value of the original block itself. By signaling to thedevice, image coding efficiency can be increased. The decoding apparatusmay derive a residual block including residual samples based on theresidual information, and generate a reconstructed block includingreconstructed samples by summing the residual block and the predictedblock, and generate a reconstructed picture including reconstructedblocks.

The residual information may be generated through transformation andquantization processes. For example, the encoding apparatus may derive aresidual block between the original block and the predicted block, andperform a transform process on residual samples (residual sample array)included in the residual block to derive transform coefficients, andthen, by performing a quantization process on the transformcoefficients, derive quantized transform coefficients to signal theresidual related information to the decoding apparatus (via abitstream). Here, the residual information may include locationinformation, a transform technique, a transform kernel, and aquantization parameter, value information of the quantized transformcoefficients etc. The decoding apparatus may performdequantization/inverse transformation process based on the residualinformation and derive residual samples (or residual blocks). Thedecoding apparatus may generate a reconstructed picture based on thepredicted block and the residual block. The encoding apparatus may alsodequantize/inverse transform the quantized transform coefficients forreference for inter prediction of a later picture to derive a residualblock, and generate a reconstructed picture based thereon.

In the present document, at least one of quantization/dequantizationand/or transform/inverse transform may be omitted. When thequantization/dequantization is omitted, the quantized transformcoefficient may be referred to as a transform coefficient. When thetransform/inverse transform is omitted, the transform coefficients maybe called coefficients or residual coefficients, or may still be calledtransform coefficients for uniformity of expression.

In the present document, a quantized transform coefficient and atransform coefficient may be referred to as a transform coefficient anda scaled transform coefficient, respectively. In this case, the residualinformation may include information on transform coefficient(s), and theinformation on the transform coefficient(s) may be signaled throughresidual coding syntax. Transform coefficients may be derived based onthe residual information (or information on the transformcoefficient(s)), and scaled transform coefficients may be derivedthrough inverse transform (scaling) on the transform coefficients.Residual samples may be derived based on an inverse transform(transform) of the scaled transform coefficients. This may beapplied/expressed in other parts of the present document as well.

Intra prediction may refer to prediction that generates predictionsamples for the current block based on reference samples in a picture towhich the current block belongs (hereinafter, referred to as a currentpicture). When intra prediction is applied to the current block,neighboring reference samples to be used for intra prediction of thecurrent block may be derived. The neighboring reference samples of thecurrent block may include samples adjacent to the left boundary of thecurrent block having a size of nW×nH and a total of 2×nH samplesneighboring the bottom-left, samples adjacent to the top boundary of thecurrent block and a total of 2×nW samples neighboring the top-right, andone sample neighboring the top-left of the current block. Alternatively,the neighboring reference samples of the current block may include aplurality of upper neighboring samples and a plurality of leftneighboring samples. In addition, the neighboring reference samples ofthe current block may include a total of nH samples adjacent to theright boundary of the current block having a size of nW×nH, a total ofnW samples adjacent to the bottom boundary of the current block, and onesample neighboring (bottom-right) neighboring bottom-right of thecurrent block.

However, some of the neighboring reference samples of the current blockmay not be decoded yet or available. In this case, the decoder mayconfigure the neighboring reference samples to use for prediction bysubstituting the samples that are not available with the availablesamples. Alternatively, neighboring reference samples to be used forprediction may be configured through interpolation of the availablesamples.

When the neighboring reference samples are derived, (i) the predictionsample may be derived based on the average or interpolation ofneighboring reference samples of the current block, and (ii) theprediction sample may be derived based on the reference sample presentin a specific (prediction) direction for the prediction sample among theperiphery reference samples of the current block. The case of (i) may becalled non-directional mode or non-angular mode and the case of (ii) maybe called directional mode or angular mode.

Furthermore, the prediction sample may also be generated throughinterpolation between the second neighboring sample and the firstneighboring sample located in a direction opposite to the predictiondirection of the intra prediction mode of the current block based on theprediction sample of the current block among the neighboring referencesamples. The above case may be referred to as linear interpolation intraprediction (LIP). In addition, chroma prediction samples may begenerated based on luma samples using a linear model. This case may becalled LM mode.

In addition, a temporary prediction sample of the current block may bederived based on filtered neighboring reference samples, and at leastone reference sample derived according to the intra prediction modeamong the existing neighboring reference samples, that is, unfilteredneighboring reference samples, and the temporary prediction sample maybe weighted-summed to derive the prediction sample of the current block.The above case may be referred to as position dependent intra prediction(PDPC).

In addition, a reference sample line having the highest predictionaccuracy among the neighboring multi-reference sample lines of thecurrent block may be selected to derive the prediction sample by usingthe reference sample located in the prediction direction on thecorresponding line, and then the reference sample line used herein maybe indicated (signaled) to the decoding apparatus, thereby performingintra-prediction encoding. The above case may be referred to asmulti-reference line (MRL) intra prediction or MRL based intraprediction.

In addition, intra prediction may be performed based on the same intraprediction mode by dividing the current block into vertical orhorizontal subpartitions, and neighboring reference samples may bederived and used in the subpartition unit. That is, in this case, theintra prediction mode for the current block is equally applied to thesubpartitions, and the intra prediction performance may be improved insome cases by deriving and using the neighboring reference samples inthe subpartition unit. Such a prediction method may be called intrasubpartitions (ISP) or ISP based intra prediction.

The above-described intra prediction methods may be called an intraprediction type separately from the intra prediction mode. The intraprediction type may be called in various terms such as an intraprediction technique or an additional intra prediction mode. Forexample, the intra prediction type (or additional intra prediction mode)may include at least one of the above-described LIP, PDPC, MRL, and ISP.A general intra prediction method except for the specific intraprediction type such as LIP, PDPC, MRL, or ISP may be called a normalintra prediction type. The normal intra prediction type may be generallyapplied when the specific intra prediction type is not applied, andprediction may be performed based on the intra prediction mode describedabove. Meanwhile, post-filtering may be performed on the predictedsample derived as needed.

Specifically, the intra prediction procedure may include an intraprediction mode/type determination step, a neighboring reference samplederivation step, and an intra prediction mode/type based predictionsample derivation step. In addition, a post-filtering step may beperformed on the predicted sample derived as needed.

When intra prediction is applied, the intra prediction mode applied tothe current block may be determined using the intra prediction mode ofthe neighboring block. For example, the decoding apparatus may selectone of most probable mode (mpm) candidates of an mpm list derived basedon the intra prediction mode of the neighboring block (ex. left and/orupper neighboring blocks) of the current block based on the received mpmindex and select one of the other remaining intro prediction modes notincluded in the mpm candidates (and planar mode) based on the remainingintra prediction mode information. The mpm list may be configured toinclude or not include a planar mode as a candidate. For example, if thempm list includes the planar mode as a candidate, the mpm list may havesix candidates. If the mpm list does not include the planar mode as acandidate, the mpm list may have three candidates. When the mpm listdoes not include the planar mode as a candidate, a not planar flag (ex.intra_luma_not_planar_flag) indicating whether an intra prediction modeof the current block is not the planar mode may be signaled. Forexample, the mpm flag may be signaled first, and the mpm index and notplanar flag may be signaled when the value of the mpm flag is 1. Inaddition, the mpm index may be signaled when the value of the not planarflag is 1. Here, the mpm list is configured not to include the planarmode as a candidate does not is to signal the not planar flag first tocheck whether it is the planar mode first because the planar mode isalways considered as mpm.

For example, whether the intra prediction mode applied to the currentblock is in mpm candidates (and planar mode) or in remaining mode may beindicated based on the mpm flag (ex. Intra_luma_mpm_flag). A value 1 ofthe mpm flag may indicate that the intra prediction mode for the currentblock is within mpm candidates (and planar mode), and a value 0 of thempm flag may indicate that the intra prediction mode for the currentblock is not in the mpm candidates (and planar mode). The value 0 of thenot planar flag (ex. Intra_luma_not_planar_flag) may indicate that theintra prediction mode for the current block is planar mode, and thevalue 1 of the not planar flag value may indicate that the intraprediction mode for the current block is not the planar mode. The mpmindex may be signaled in the form of an mpm_idx or intra_luma_mpm_idxsyntax element, and the remaining intra prediction mode information maybe signaled in the form of a rem_intra_luma_pred_mode orintra_luma_mpm_remainder syntax element. For example, the remainingintra prediction mode information may index remaining intra predictionmodes not included in the mpm candidates (and planar mode) among allintra prediction modes in order of prediction mode number to indicateone of them. The intra prediction mode may be an intra prediction modefor a luma component (sample). Hereinafter, intra prediction modeinformation may include at least one of the mpm flag (ex.Intra_luma_mpm_flag), the not planar flag (ex.Intra_luma_not_planar_flag), the mpm index (ex. mpm_idx orintra_luma_mpm_idx), and the remaining intra prediction mode information(rem_intra_luma_pred_mode or intra_luma_mpm_remainder). In the presentdocument, the MPM list may be referred to in various terms such as MPMcandidate list and candModeList. When MIP is applied to the currentblock, a separate mpm flag (ex. intra_mip_mpm_flag), an mpm index (ex.intra_mip_mpm_idx), and remaining intra prediction mode information (ex.intra_mip_mpm_remainder) for MIP may be signaled and the not planar flagis not signaled.

In other words, in general, when block splitting is performed on animage, a current block and a neighboring block to be coded have similarimage characteristics. Therefore, the current block and the neighboringblock have a high probability of having the same or similar intraprediction mode. Thus, the encoder may use the intra prediction mode ofthe neighboring block to encode the intra prediction mode of the currentblock.

For example, the encoder/decoder may configure a list of most probablemodes (MPM) for the current block. The MPM list may also be referred toas an MPM candidate list. Herein, the MPM may refer to a mode used toimprove coding efficiency in consideration of similarity between thecurrent block and neighboring block in intra prediction mode coding. Asdescribed above, the MPM list may be configured to include the planarmode or may be configured to exclude the planar mode. For example, whenthe MPM list includes the planar mode, the number of candidates in theMPM list may be 6. And, if the MPM list does not include the planarmode, the number of candidates in the MPM list may be 5.

The encoder/decoder may configure an MPM list including 5 or 6 MPMs.

In order to configure the MPM list, three types of modes can beconsidered: default intra modes, neighbor intra modes, and the derivedintra modes.

For the neighboring intra modes, two neighboring blocks, i.e., a leftneighboring block and an upper neighboring block, may be considered.

As described above, if the MPM list is configured not to include theplanar mode, the planar mode is excluded from the list, and the numberof MPM list candidates may be set to 5.

In addition, the non-directional mode (or non-angular mode) among theintra prediction modes may include a DC mode based on the average ofneighboring reference samples of the current block or a planar modebased on interpolation.

When inter prediction is applied, the predictor of the encodingapparatus/decoding apparatus may derive a prediction sample byperforming inter prediction in units of blocks. Inter prediction may bea prediction derived in a manner that is dependent on data elements (ex.sample values or motion information) of picture(s) other than thecurrent picture. When inter prediction is applied to the current block,a predicted block (prediction sample array) for the current block may bederived based on a reference block (reference sample array) specified bya motion vector on the reference picture indicated by the referencepicture index. Here, in order to reduce the amount of motion informationtransmitted in the inter prediction mode, the motion information of thecurrent block may be predicted in units of blocks, subblocks, or samplesbased on correlation of motion information between the neighboring blockand the current block. The motion information may include a motionvector and a reference picture index. The motion information may furtherinclude inter prediction type (L0 prediction, L1 prediction, Biprediction, etc.) information. In the case of inter prediction, theneighboring block may include a spatial neighboring block present in thecurrent picture and a temporal neighboring block present in thereference picture. The reference picture including the reference blockand the reference picture including the temporal neighboring block maybe the same or different. The temporal neighboring block may be called acollocated reference block, a co-located CU (colCU), and the like, andthe reference picture including the temporal neighboring block may becalled a collocated picture (colPic). For example, a motion informationcandidate list may be configured based on neighboring blocks of thecurrent block, and flag or index information indicating which candidateis selected (used) may be signaled to derive a motion vector and/or areference picture index of the current block. Inter prediction may beperformed based on various prediction modes. For example, in the case ofa skip mode and a merge mode, the motion information of the currentblock may be the same as motion information of the neighboring block. Inthe skip mode, unlike the merge mode, the residual signal may not betransmitted. In the case of the motion vector prediction (MVP) mode, themotion vector of the selected neighboring block may be used as a motionvector predictor and the motion vector of the current block may besignaled. In this case, the motion vector of the current block may bederived using the sum of the motion vector predictor and the motionvector difference.

The motion information may include L0 motion information and/or L1motion information according to an inter prediction type (L0 prediction,L1 prediction, Bi prediction, etc.). The motion vector in the L0direction may be referred to as an L0 motion vector or MVL0, and themotion vector in the L1 direction may be referred to as an L1 motionvector or MVL1. Prediction based on the L0 motion vector may be calledL0 prediction, prediction based on the L1 motion vector may be called L1prediction, and prediction based on both the L0 motion vector and the L1motion vector may be called bi-prediction. Here, the L0 motion vectormay indicate a motion vector associated with the reference picture listL0 (L0), and the L1 motion vector may indicate a motion vectorassociated with the reference picture list L1 (L1). The referencepicture list L0 may include pictures that are earlier in output orderthan the current picture as reference pictures, and the referencepicture list L1 may include pictures that are later in the output orderthan the current picture. The previous pictures may be called forward(reference) pictures, and the subsequent pictures may be called reverse(reference) pictures. The reference picture list L0 may further includepictures that are later in the output order than the current picture asreference pictures. In this case, the previous pictures may be indexedfirst in the reference picture list L0 and the subsequent pictures maybe indexed later. The reference picture list L1 may further includeprevious pictures in the output order than the current picture asreference pictures. In this case, the subsequent pictures may be indexedfirst in the reference picture list 1 and the previous pictures may beindexed later. The output order may correspond to picture order count(POC) order.

FIG. 4 shows a block diagram of context-adaptive arithmetic coding(CABAC).

The encoding process of CABAC first converts an input signal into abinary value through binarization when the input signal is a syntaxelement rather than a binary value. When the input signal is already abinary value, it may be bypassed without binarization. Here, each binarynumber 0 or 1 constituting a binary value is called a bin. For example,when a binary string (a bin string) after binarization is 110, each of1, 1, and 0 is called one bin. The bin(s) for one syntax element mayindicate the value of the corresponding syntax element.

The binarized bins are input to a regular coding engine or a bypasscoding engine. The regular coding engine allocates a context model thatreflects a probability value to the corresponding bin, and encodes thecorresponding bin based on the allocated context model.

In the regular coding engine, after coding for each bin, theprobabilistic model for the corresponding bin may be updated. Bins codedin this way are called context-coded bins. The bypass coding engineomits a procedure of estimating a probability for an input bin and aprocedure of updating a probabilistic model applied to the correspondingbin after coding. The coding speed is improved by coding the input binby applying a uniform probability distribution (e.g., 50:50) instead ofallocating a context. Bins coded in this way are called bypass bins. Thecontext model may be allocated and updated for each bin to be contextcoded (regular coded), and the context model may be indicated based onctxidx or ctxInc. ctxidx may be derived based on ctxInc. Specifically,for example, the context index (ctxidx) indicating the context model foreach of the regularly coded bins may be derived as the sum of thecontext index increment (ctxInc) and the context index offset(ctxIdxOffset). Here, the ctxInc may be derived differently for eachbin. The ctxIdxOffset may be represented by the lowest value of thectxIdx. The lowest value of the ctxIdx may be referred to as an initialvalue (initValue) of the ctxIdx. The ctxIdxOffset is a value generallyused to distinguish context models for other syntax elements, and acontext model for one syntax element may be distinguished/derived basedon the ctxinc.

In the entropy encoding procedure, it may be determined whether toperform encoding through the regular coding engine or to performencoding through the bypass coding engine, and accordingly, a codingpath may be switched. Entropy decoding may perform the same process asthe entropy encoding in reverse order.

FIG. 5 shows an entropy encoder of an encoding apparatus.

An entropy encoder 500 of FIG. 5 may be the same as the entropy encoder240 of FIG. 2 described above. A binarization unit 501 in the entropyencoder 500 may perform binarization on a target syntax element. Here,the binarization may be based on various binarization methods such as atruncated rice binarization process and a fixed-length binarizationprocess, and the binarization method for a target syntax element may bepre-defined.

An entropy encoding processor 502 in the entropy encoder 500 may performentropy encoding on the target syntax element. The encoding apparatusmay encode an empty string of a target syntax element based on regularcoding (context based) or bypass coding based on an entropy codingtechnique such as context-adaptive arithmetic coding (CABAC) orcontext-adaptive variable length coding (CAVLC), and the output may beincluded in the bitstream. As described above, the bitstream may betransmitted to a decoding apparatus through a (digital) storage mediumor a network.

FIG. 6 shows an entropy decoder of a decoding apparatus.

An entropy decoder 600 of FIG. 6 may be the same as the entropy decoder310 of FIG. 3 described above. A binarization unit 601 in the entropyencoder 600 may perform binarization on a target syntax element. Here,the binarization may be based on various binarization methods such as atruncated rice binarization process and a fixed-length binarizationprocess, and a binarization method for a target syntax element may bepredefined. The decoding apparatus may derive available bin strings (binstring candidates) for available values of the target syntax elementthrough the binarization process.

An entropy decoding processor 602 in the entropy decoder 600 may performentropy decoding on the target syntax element. The decoding apparatussequentially decodes and parses each bin for the target syntax elementfrom the input bit(s) in the bitstream, and compares the derived binstring with available bin strings for the corresponding syntax element.If the derived bin string is the same as one of the available binstrings, a value corresponding to the bin string is derived as the valueof the corresponding syntax element. If not, the above-described processis performed again after further parsing the next bit in the bitstream.Through this process, the corresponding information may be signaledusing a variable length bit without using a start bit or an end bit forspecific information (a specific syntax element) in the bitstream.Through this, relatively fewer bits may be allocated to a low value, andoverall coding efficiency can be increased.

FIG. 7 exemplarily shows a hierarchical structure for a codedimage/video.

Referring to FIG. 7 , coded image/video is divided into a video codinglayer (VCL) that handles the decoding process of the image/video anditself, a subsystem that transmits and stores the coded information, andNAL (network abstraction layer) in charge of function and presentbetween the VCL and the subsystem.

In the VCL, VCL data including compressed image data (slice data) isgenerated, or a parameter set including a picture parameter set (PSP), asequence parameter set (SPS), and a video parameter set (VPS) or asupplemental enhancement information (SEI) message additionally requiredfor an image decoding process may be generated.

In the NAL, a NAL unit may be generated by adding header information(NAL unit header) to a raw byte sequence payload (RBSP) generated in aVCL. In this case, the RBSP refers to slice data, parameter set, SEImessage, etc., generated in the VCL. The NAL unit header may include NALunit type information specified according to RBSP data included in thecorresponding NAL unit.

As shown in the figure, the NAL unit may be classified into a VCL NALunit and a Non-VCL NAL unit according to the RBSP generated in the VCL.The VCL NAL unit may mean a NAL unit that includes information on theimage (slice data) on the image, and the Non-VCL NAL unit may mean a NALunit that includes information (parameter set or SEI message) requiredfor decoding the image.

The above-described VCL NAL unit and Non-VCL NAL unit may be transmittedthrough a network by attaching header information according to the datastandard of the subsystem. For example, the NAL unit may be transformedinto a data format of a predetermined standard such as an H.266/VVC fileformat, a real-time transport protocol (RTP), a transport stream (TS),etc., and transmitted through various networks.

As described above, the NAL unit may be specified with the NAL unit typeaccording to the RBSP data structure included in the corresponding NALunit, and information on the NAL unit type may be stored and signaled inthe NAL unit header.

For example, the NAL unit may be classified into a VCL NAL unit type anda Non-VCL NAL unit type according to whether the NAL unit includesinformation (slice data) about an image. The VCL NAL unit type may beclassified according to the nature and type of pictures included in theVCL NAL unit, and the Non-VCL NAL unit type may be classified accordingto types of parameter sets.

The following is an example of the NAL unit type specified according tothe type of parameter set included in the Non-VCL NAL unit type.

-   -   APS (Adaptation Parameter Set) NAL unit: Type for NAL unit        including APS    -   DPS (Decoding Parameter Set) NAL unit: Type for NAL unit        including DPS    -   VPS (Video Parameter Set) NAL unit: Type for NAL unit including        VPS    -   SPS(Sequence Parameter Set) NAL unit: Type for NAL unit        including SPS    -   PPS (Picture Parameter Set) NAL unit: Type for NAL unit        including PPS    -   PH (Picture header) NAL unit: Type for NAL unit including PH

The aforementioned NAL unit types may have syntax information for theNAL unit type, and the syntax information may be stored and signaled ina NAL unit header. For example, the syntax information may benal_unit_type, and NAL unit types may be specified by a nal_unit_typevalue.

Meanwhile, as described above, one picture may include a plurality ofslices, and one slice may include a slice header and slice data. In thiscase, one picture header may be further added to a plurality of slices(a slice header and a slice data set) in one picture. The picture header(picture header syntax) may include information/parameters commonlyapplicable to the picture. In the present document, a tile group may bemixed or replaced with a slice or a picture. Also, in the presentdocument, a tile group header may be mixed or replaced with a sliceheader or a picture header.

The slice header (slice header syntax) may includeinformation/parameters that may be commonly applied to the slice. TheAPS (APS syntax) or the PPS (PPS syntax) may includeinformation/parameters that may be commonly applied to one or moreslices or pictures. The SPS (SPS syntax) may includeinformation/parameters that may be commonly applied to one or moresequences. The VPS (VPS syntax) may include information/parameters thatmay be commonly applied to multiple layers. The DPS (DPS syntax) mayinclude information/parameters that may be commonly applied to theoverall video. The DPS may include information/parameters related toconcatenation of a coded video sequence (CVS). The high level syntax(HLS) in the present document may include at least one of the APSsyntax, the PPS syntax, the SPS syntax, the VPS syntax, the DPS syntax,and the slice header syntax.

In the present document, the image/image information encoded from theencoding apparatus and signaled to the decoding apparatus in the form ofa bitstream includes not only partitioning related information in apicture, intra/inter prediction information, residual information,in-loop filtering information, etc., but also information included in aslice header, information included in the APS, information included inthe PPS, information included in an SPS, and/or information included inthe VPS.

Meanwhile, in order to compensate for a difference between an originalimage and a reconstructed image due to an error occurring in acompression coding process such as quantization, an in-loop filteringprocess may be performed on reconstructed samples or reconstructedpictures as described above. As described above, the in-loop filteringmay be performed by the filter of the encoding apparatus and the filterof the decoding apparatus, and a deblocking filter, SAO, and/or adaptiveloop filter (ALF) may be applied. For example, the ALF process may beperformed after the deblocking filtering process and/or the SAO processare completed. However, even in this case, the deblocking filteringprocess and/or the SAO process may be omitted.

FIG. 8 is a flowchart schematically illustrating an example of an ALFprocess. The ALF process disclosed in FIG. 8 may be performed in anencoding apparatus and a decoding apparatus. In this document, thecoding apparatus may include the encoding apparatus and/or the decodingapparatus.

Referring to FIG. 8 , the coding apparatus derives a filter for ALF(S800). The filter may include filter coefficients. The coding apparatusmay determine whether to apply the ALF, and when determining to applythe ALF, may derive a filter including filter coefficients for the ALF.Information for deriving a filter (coefficients) for ALF or a filter(coefficients) for ALF may be referred to as an ALF parameter.Information on whether ALF is applied (i.e., ALF enabled flag) and ALFdata for deriving the filter may be signaled from the encoding apparatusto the decoding apparatus. ALF data may include information for derivinga filter for the ALF. Also, for example, for hierarchical control ofALF, an ALF enabled flag may be signaled at the SPS, picture header,slice header, and/or CTB level, respectively.

In order to derive the filter for the ALF, the activity and/ordirectivity of the current block (or ALF target block) is derived, andthe filter may be derived based on the activity and/or thedirectionality. For example, the ALF process may be applied in units of4×4 blocks (based on luma components). The current block or the ALFtarget block may be, for example, a CU, or may be a 4×4 block within aCU. Specifically, for example, filters for ALF may be derived based onfirst filters derived from information included in the ALF data andpredefined second filters, and the coding apparatus may select one ofthe filters based on the activity and/or the directionality. The codingapparatus may use filter coefficients included in the selected filterfor the ALF.

The coding apparatus performs filtering based on the filter (S810).Modified reconstructed samples may be derived based on the filtering.For example, the filter coefficients in the filter may be arranged orallocated according to a filter shape, and the filtering may beperformed on reconstructed samples in the current block. Here, thereconstructed samples in the current block may be reconstructed samplesafter the deblocking filter process and the SAO process are completed.For example, one filter shape may be used, or one filter shape may beselected and used from among a plurality of predetermined filter shapes.For example, a filter shape applied to the luma component and a filtershape applied to the chroma component may be different. For example, a7×7 diamond filter shape may be used for the luma component, and a 5×5diamond filter shape may be used for the chroma component.

FIG. 9 shows examples of the shape of an ALF filter. C0˜C11 in (a) ofFIG. 9 and C0˜C5 in (b) of FIG. 9 may be filter coefficients dependingon positions within each filter shape.

(a) of FIG. 9 shows the shape of a 7×7 diamond filter, and (b) of FIG. 9shows the shape of a 5×5 diamond filter. In FIG. 9 , Cn in the filtershape represents a filter coefficient. When n in Cn is the same, thisindicates that the same filter coefficients can be assigned. In thepresent document, a position and/or unit to which filter coefficientsare assigned according to the filter shape of the ALF may be referred toas a filter tab. In this case, one filter coefficient may be assigned toeach filter tap, and the arrangement of the filter taps may correspondto a filter shape. A filter tab located at the center of the filtershape may be referred to as a center filter tab. The same filtercoefficients may be assigned to two filter taps of the same n value thatexist at positions corresponding to each other with respect to thecenter filter tap. For example, in the case of a 7×7 diamond filtershape, 25 filter taps are included, and since filter coefficients C0 toC11 are assigned in a centrally symmetric form, filter coefficients maybe assigned to the 25 filter taps using only 13 filter coefficients.Also, for example, in the case of a 5×5 diamond filter shape, 13 filtertaps are included, and since filter coefficients C0 to C5 are assignedin a centrally symmetrical form, filter coefficients may be assigned tothe 13 filter taps using only 7 filter coefficients. For example, inorder to reduce the data amount of signaled information on filtercoefficients, 12 filter coefficients among 13 filter coefficients forthe 7×7 diamond filter shape may be signaled (explicitly), and onefilter coefficient may be derived (implicitly). Also, for example, 6filter coefficients among 7 filter coefficients for a 5×5 diamond filtershape may be signaled (explicitly) and one filter coefficient may bederived (implicitly).

According to an embodiment of the present document, the ALF parameterused for the ALF process may be signaled through an adaptation parameterset (APS). The ALF parameter may be derived from filter information forthe ALF or ALF data.

The ALF is a type of in-loop filtering technique that can be applied invideo/image coding as described above. The ALF may be performed using aWiener-based adaptive filter. This may be to minimize a mean squareerror (MSE) between original samples and decoded samples (orreconstructed samples). A high level design for an ALF tool mayincorporate syntax elements accessible from the SPS and/or slice header(or tile group header).

FIG. 10 shows an example of a hierarchical structure of ALF data.

Referring to FIG. 10 , a coded video sequence (CVS) 900 may include anSPS, one or more PPSs, and one or more coded pictures that follow. Eachcoded picture may be divided into rectangular regions. The rectangularregions may be referred to as tiles. One or more tiles may be aggregatedto form a tile group or slice. In this case, the tile group header maybe linked to the PPS, and the PPS may be linked to the SPS. According tothe existing method, the ALF data (ALF parameter) is included in thetile group header. Considering that one video consists of a plurality ofpictures and one picture includes a plurality of tiles, the frequent ALFdata (ALF parameter) signaling in units of tile groups reduces codingefficiency.

According to an embodiment proposed in the present disclosure, the ALFparameter may be included in the APS and signaled as follows.

FIG. 11 shows another example of a hierarchical structure of ALF data.

Referring to FIG. 11 , a CVS 1100 may include an SPS, one or more PPSs,one or more APSs, and one or more coded pictures that follow. That is,an APS is defined, and the APS may carry necessary ALF data (ALFparameters). In addition, the APS may have self-identificationparameters and ALF data. The self-identification parameter of the APSmay include an APS ID. That is, the APS may include informationindicating the APS ID in addition to the ALF data field. The tile groupheader or the slice header may refer to the APS using APS indexinformation. In other words, the tile group header or the slice headermay include APS index information, and the ALF process for the targetblock may be performed based on the ALF data (ALF parameter) included inthe APS having the APS ID indicated by the APS index information. Here,the APS index information may be referred to as APS ID information.

In addition, the SPS may include a flag allowing the use of the ALF. Forexample, when CVS begins, an SPS may be checked, and the flag may bechecked in the SPS. For example, the SPS may include the syntax of Table1 below. The syntax of Table 1 may be a part of the SPS.

TABLE 2 seq_parameter_set_rbsp( ) { Descriptor . . . sps_alf_enabled_flag u(1) }

The semantics of syntax elements included in the syntax of Table 1 maybe expressed, for example, as shown in Table 2 below.

TABLE 2 sps_alf_enabled_flag equal to 0 specifies that the adaptive loopfilter is disabled. sps_alf_enabled_flag equal to 1 specifies that theadaptive loop filter is enabled.

That is, the syntax element sps_alf_enabled_flag may indicate whetherALF is enabled. When the value of the ALF enabled flagsps_alf_enabled_flag signaled by the SPS (or SPS level) is 1, it may bedetermined that the ALF is enabled for pictures in the CVS referring tothe SPS. In addition, when the value of sps_alf_enabled_flag is 0, itmay be determined that the ALF is not enabled for pictures in the CVSreferring to the SPS.

Meanwhile, as described above, the ALF may be individually turned on/offby signaling an additional enabled flag at a lower level than the SPS.For example, when the ALF tool is enabled for CVS,slice_alf_enabled_flag may be signaled in a tile group header or a sliceheader. For example, the slice header may include the syntax of Table 3below.

TABLE 3 slice_header( ) { Descriptor  slice_pic_parameter_set_id ue(v) if( sps_alf_enabled_flag ) {   slice_alf_enabled_flag u(1)  if(slice_alf_enabled_flag )      num_alf_aps_ids tb(v)      for(i = 0;i<num_alf_aps_ids:i++)    slice_alf_aps_id_luma[i] u(5)      slice_alf_chroma_idc tu(v)     if(slice_alf_aps _chroma_idc &&(slice_type != I || num_alf_aps_ids != 1))      slice_alf_aps_id_chormau(5)  }

Semantics of syntax elements included in the syntax of Table 3 may beexpressed, for example, as shown in the following table.

TABLE 4 slice_alf_enabled_flag equal to 1 specifies that adaptive loopfilter is enabled and may be applied to Y, Cb, or Cr color component ina slice. slice_alf_enabled_flag equal to 0 specifies that adaptive loopfilter is disabled for all color components in a tile group.num_alf_aps_ids specifies the number of ALF APSs that the slice refersto. The value  of num_alf_aps_ids shall be in the range of 0 to 6,inclusive. The maximum value maxVal of the truncated binary binarizationtb(v) is set equal to 1 for intra slices and slices in an IRAP picture,and set equal to 6 otherwise. slice_alf_aps_id_luma[i] specifies theadaptation_parameter_set_id of the APS that the slice refers to. TheTemporalId of the APS NAL unit having adaptation_parameter_set_ id equalto tile_group_aps_id shall be less than or equal to the TemporalId ofthe coded tile group NAL unit. When multiple APSs with the same value ofadaptation_parameter_set_id are referred to by two of more tile groupsof the same picture, the multiple APSs with the same value ofadaptation_parameter_set_id shall have^(:) the same content.slice_alf_chroma_idc equal to 0 specifies that the adaptive loop filteris not applied to Cb and Cr colour components, slice_alf_chroma_idcequal to 1 indicates that the adaptive loop filter is applied to the Cbcolour component. slice_alf_chroma_idc equal to 2 indicates that theadaptive loop filter is applied to the Cr colour component.slice_alf_chroma_idc equal to 3 indicates that the adaptive loop filteris applied to Cb and Cr colour components. When slice_alf chroma_idc isnot present, it is inferred to be equal to 0. The maximum value maxValof the trauncated unary binarization tu(v) is set equal to 3.slice_alf_aps_id_chroma specifies the adaptation_parameter_set_id thatthe chroma component of the slice refers to. Whenslice_alf_aps_id_chroma is not present, it is inferred to be equal toslice_alf_aps_id _luma[ 0 ]. The TemporalId of the ALF APS NAL unithaving adaptation_parameter_set_id equal to slice_alf_aps_id_chromashall be less than or equal to the TemporalId of the coded slice NALunit. For intra slices and slices in an IRAP picture,slice_alf_aps_id_chroma shall not refer to an ALF APS associated withother pictures rather than the picture containing the intra slices orthe IRAP picture.

The slice_alf_enabled_flag may be parsed/signaled when, for example, ALFis enabled at the SPS level. Each tile group composed of one or moretiles may determine whether ALF is enabled based onslice_alf_enabled_flag.

When the value of the ALF enabled flag sps_alf_enabled_flag signaled ina tile group header or a slice header is 1, ALF data may be parsedthrough the slice header. For example, the slice_alf_enabled_flag mayspecify an ALF enabling condition for luma and chroma components.

The ALF data may be accessed through APS ID information, that is,slice_alf_aps_id. An APS referenced by a corresponding tile group or acorresponding slice may be identified based on the APS ID information.The APS may include ALF data.

Meanwhile, the structure of the APS including the ALF data may bedescribed based on, for example, the syntax of Table 5 below. The syntaxof Table 5 may be a part of the APS.

TABLE 5 adaptation_parameter_set_rbsp( ) { Descriptor adaptation_parameter_set_id u(5)  alf_data( )  aps_extension_flag u(1) if( aps_extension_flag)   while( more_rbsp_data( ) )   aps_extension_data_flag u(1)  rbsp_trailing_bits( ) }

Semantics of syntax elements included in the syntax of Table 5 may beexpressed, for example, as shown in Table 6 below.

TABLE 6 adaptation_parameter_set_id provides an identifier for the APSfor reference by other syntax elements. NOTE - APSs can be shared acrosspictures and can be different in different tile groups within a picture.aps_extension_flag equal to 0 specifies that no aps_extension_data_flagsyntax elements are present in the APS RBSP syntax structure.aps_extension_flag equal to 1 specifies that there areaps_extension_data_flag syntax elements present in the APS RBSP syntaxstructure. aps_extension_data_flag may have any value. Its presence andvalue do not affect decoder conformance to profiles specified in thisversion of this Specification Decoders conforming to this version ofthis Specification shall ignore all aps_extension_data_flag syntaxelements.

As described above, the syntax element adaptation_parameter_set_id mayindicate the identifier of the corresponding APS. That is, the APS maybe identified based on the syntax element adaptation_parameter_set_id.The syntax element adaptation_parameter_set_id may be referred to as APSID information. Also, the APS may include an ALF data field. The ALFdata field alf_data( ) may be parsed/signaled after the syntax elementadaptation_parameter_set_id.

Core processing/handling of ALF information may be performed in a sliceheader or a tile group header. The above described ALF data field mayinclude information on processing of the ALF filter. For example,information that can be extracted from the ALF data field includesinformation on the number of filters used, information indicatingwhether the ALF is applied only to a luma component, information on acolor component, information on an exponential golomb (EG) parametersand/or delta values of filter coefficients, etc.

Meanwhile, the ALF data field may include, for example, ALF data syntaxas follows.

TABLE 7 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1)if( alf_luma_filter_signal_flag ) {   alf_luma_clip_flag u(1) alf_luma_num_filters_signalled_minus1 tb(v)  if(alf_luma_num_filters_signalled_minus1 > 0 ) {   for( filtIdx = 0:filtIdx < NumAlfFilters; filtIdx++ )    alf_luma_coeff_delta_idx[filtIdx ] tb(v)  }   alf_luma_use_fixed_filter_flag u(1)   if(alf_luma_use_fixed_filter_flag ) {    alf_luma_fixed_filter_set_idxtb(v)    alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag ) (     for( i = 0: i <NumAlfFilters; i++ )      alf_luma_fixed_filter_pred_flag[ i ] u(1)    }  }  alf_luma_coeff_delta_flag u(1)  if ( !alf_luma_coeff_delta_flag &&alf_luma_num_filters_signalled_minus1 > 0 )  alf_luma_coeff_delta_prediction_flag u(1) alf_luma_min_eg_order_minus1 ue(v)  for( i = 0; i < 3; i++ )  alf_luma_eg_order_increase_flag[ i ] u(1) if(alf_luma_coeff_delta_flag ) {   for( sIdx = 0; sIdx <=alf_luma_num_filters_signalled_minus1; sIdx++ )    alf_luma_coeff_flag[sIdx ] u(1)  }  for( sIdx = 0; sIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {   if(alf_luma_coeff_flag[ sfIdx ] ) {    for ( j = 0; j < 12; j++ ) {    alf_luma_coeff_delta_abs[ sIdx ][ j ] uek(v)     if(alf_luma_coeff_delta_abs[ sIdx ][ j ] )      alf_luma_coeff_delta_sign[sIdx ][ j ] u(1)    }   }  }   if( alf_luma_clip_flag ) {   alf_luma_clip_min_eg_order_minus1 ue(v)    for( i = 0; i < 3; i++ )    alf_luma_clip_eg_order_increase_flag[ i ] u(1)    for( sfIdx = 0;sfIdx <= alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     if(alf_luma_coeff_flag[ sfIdx ] ) {      for( j = 0: j < 12; j++ ) {      if( filtCoeff[ sfIdx ][ j ]        alf_luma_clip_idx[ sfIdx ][ j ]uek(v)      }     }    }   }  }  if( alf_chroma_filter_signal_flag > 0 ){ u(1)   alf_chroma_clip_flag   alf_chroma_min_eg_order_minus1 ue(v)  for(i = 0; i < 2; i++ ) { u(1)    alf_chroma_eg_order_increase_flag[ i]   for( j = 0: j < 6: j++ ) {    alf_chroma_coeff_abs[ i ] uek(v)   if( alf_chroma_coeff_abs[ j ] > 0 )     alf_chroma_coeff_sign[ j ]u(1)   }   if( alf_chroma_clip_flag ) {   alf_chroma_clip_min_eg_order_minus1 ue(v)    for( i = 0; i < 2; i++ )    alf_chroma_clip_eg_order_increase_flag[ i ] u(1)    for( j = 0; j <6; j++ ) {     if( alf_chroma_coeff_abs[ j ] )      alf_chroma_clip_idx[j ] uek(v)    }   }  } }

Semantics of syntax elements included in the syntax of Table 7 may beexpressed, for example, as shown in the following table.

Parsing of ALF data in the slice header is started by firstparsing/signaling a syntax element alf_luma_filter_signal_flagindicating whether a luma filter set is signaled. Similarly, the syntaxelement alf_chroma_filter_signal_flag indicating whether a chroma filteris signaled may be parsed/signaled.

When alf_luma_filter_signal_flag is enabled, alf_luma_clip_flag may beparsed.

When this flag is disabled, it may specify that linear adaptive loopfiltering is applied to the luma component. When this flag is enabled,it may specify that non-linear adaptive loop filtering is applied to theluma component.

In addition, information on the number of used luma filters may beparsed.

As an example, the maximum number of filters that may be used may be setto 25. When the number of signaled luma filters is at least one, foreach filter in the range from 0 to the maximum number of filters (i.e.,25, which may alternatively be known as class), index information forthe filter may be parsed/signaled. This may imply that every class(i.e., from 0 to the maximum number of filters) is associated with afilter index.

Next, alf_luma_use_fixed_filter_flag, which is a flag indicating whethera fixed filter is used in the current slice, may be parsed/signaled.When alf_luma_use_fixed_filter_flag is enabled, an index for the fixedfilter set may be signaled using the alf_luma_fixed_filter_set_idxparameter. The parameter may have values ranging from 0 to 15. Inaddition, alf_luma_fixed_filter_pred_present_flag may beparsed/signaled. This flag may specify whether to predict the i-thfilter coefficient using a fixed filter. Then, for each filter, a flagindicating whether a fixed filter is used for the corresponding filtermay be decoded.

When a filter to be used for each class is labeled based on the filterindex, alf_luma_coeff_delta_flag may be parsed/signaled. The flag may beused to interpret whether flag, alf_luma_coeff_delta_prediction_flagrelated to prediction of an ALF luma filter coefficient delta valueexists in a slice header.

If the number of luma filters signaled by the syntax elementalf_luma_num_filters_signalled_minus1 is greater than 0 and the value ofthe syntax element alf_luma_coeff_delta_flag is 0, it means that thesyntax element alf_luma_coeff_delta_prediction_flag is present in theslice header and its status is evaluated. If the state of the syntaxelement alf_luma_coeff_delta_prediction_flag indicates 1, this may meanthat luma filter coefficients are predicted from previous luma (filter)coefficients. If the state of the syntax elementalf_luma_coeff_delta_prediction_flag indicates 0, this may mean thatluma filter coefficients are not predicted from deltas of previous luma(filter) coefficients.

Furthermore, when delta filter coefficients (i.e.,alf_luma_coeff_delta_abs) are coded based on the exponential Golombcode, order k (order-k) of the exponential Golomb (EG) code may bedetermined in order to decode the delta luma filter coefficients (i.e.,alf_luma_coeff_delta_abs). For example, filter coefficients may bedecoded based on exponential Golomb coding using order k. The order ofthe exponential Golomb code may be expressed as EG(k).

In order to determine the EG(k), the syntax elementalf_luma_min_eg_order_minus1 may be parsed/signaled. The syntax elementalf_luma_min_eg_order_minus1 may be an entropy-coded syntax element. Thesyntax element alf_luma_min_eg_order_minus1 may indicate a smallestorder of an EG used for decoding the delta luma filter coefficients. Forexample, the value of the syntax element alf_luma_min_eg_order_minus1may be in the range of 0 to 6.

After the syntax element alf_luma_min_eg_order_minus1 isparsed/signaled, the syntax element alf_luma_eg_order_increase_flag maybe parsed/signaled. If the value of the syntax elementalf_luma_eg_order_increase_flag is 1, this indicates that the order ofthe EG indicated by the syntax element alf_luma_min_eg_order_minus1increases by 1. If the value of the syntax elementalf_luma_eg_order_increase_flag is 0, this indicates that the order ofthe EG indicated by the syntax element alf_luma_min_eg_order_minus1 doesnot increase. The order of the EG may be represented by an index of theEG. The EG order (or EG index) (related to the luma component) based onthe syntax element alf_luma_min_eg_order_minus1 and the syntax elementalf_luma_eg_order_increase_flag may be determined, for example, asfollows.

TABLE 9 1. The maximum golomb index is pre-defined according to thefilter type. That is for a 5 × 5 filter the maximum golomb index is 2,and for a 7 × 7 the maximum golomb index is 3. 2. An initial constant,Kmin is set to be alf_luma_min_eg_order_minus1 + 1 3. For each index, iin the ange of {0 to maximun golomb index}, an intermediate array, i.e.,KminTab[i] is computed, consisting of the sum of    KminTab[i] = Kmin +alf_luma_eg_order_incrcase_flag[i] 4. Kmin is refreshed at the end ofeach iteration to be       Kmin = KminTab[i]

Based on the determination process, expGoOrderY may be derived asexpGoOrderY=KminTab. Through this, an array including EG orders can bederived, which can be used by the decoding apparatus. The expGoOrderYmay indicate the EG order (or EG index).

The table below may represent a method for deriving the Golomb orderbased on the above-described algorithm. That is, the above-describedalgorithm may be based on the table below. In the table below, theGolomb index may be the input, and the k order to be used may be thecorresponding output.

TABLE 10 Golomb Golomb Golomb index = 0 index = 1 index = 2 Kmin = 1 1 12 alf_luma_eg_order_increase_flag[ i ] 0 1 1 Golomb order-k 1 2 3

Referring to Table 10, the (minimum) Golomb order and/or the (minimum)Golomb index may be increased based on alf_eg_order_increase_flag[i].For example, if alf_luma_min_eg_order_minus1=0, Kmin may be set to =0+1.When the Golomb index is 0, alf_luma_eg_order_increase_flag[i] may beparsed. When the flag is enabled (if available), the Golomb ordercorresponding to the Golomb index=0 may increase by 1 from the initialKmin value. As shown in Table 11, when the flag is disabled (notavailable) when the Golomb index=0, the Golomb order may not increase.Kmin may be updated with the Golomb order and for example the Golombindex may be treated (updated) as 1. Here, the value ofalf_luma_eg_order_increase_flag[1] may be 1. Therefore, the Kmin Golomborder can be increased to 2. Similarly,alf_luma_eg_order_increase_flag[2] is enabled (available), which mayindicate that Kmin is increased by 1 to make the final golomb order 3.

FIG. 12 exemplarily shows Golomb indexes of filter coefficientsaccording to types of filters. (a) of FIG. 12 may represent a 5×5filter, and (b) of FIG. 12 may represent a 7×7 filter. Here, only a partof the symmetric filter can be shown. In FIG. 12 , the numbers of eachposition may represent pre-assigned Golomb indices for ALF filtercoefficients corresponding to each position.

In one example, the kth-order Golomb order for a given filtercoefficient may be determined by indexing the Golomb index of the givencoefficient position into Table 11 above. Each ALF filter coefficientcan be decoded using the k-th order.

In one example, there may be a pre-defined Golomb order index (i.e.,golombOrderIdxY). The pre-defined Golomb order may be used to determinea final golomb order for coding the coefficients.

For example, the pre-defined Golomb order may be configured as, forexample, the following equation.golombOrderIdxY[ ]=0,0,1,0,1,2,1,0,0,1,2  [Equation 1]

Here, the order k=expGoOrderY[golombOrderIdxY[j]], and j may representthe j-th signaled filter coefficient. For example, if j=2, that is, thethird filter coefficient, golomborderIdxY[2]=1, and thusk=expGoOrderY[1].

In this case, for example, if the value of the syntax elementalf_luma_coeff_delta_flag represents true, that is, 1, the syntaxelement alf_luma_coeff_flag may be signaled for every filter that issignaled. The syntax element alf_luma_coeff_flag indicates whether aluma filter coefficient is (explicitly) signaled.

When the EG order and the states of the aforementioned related flags(i.e., alf_luma_coeff_delta_flag, alf_luma_coeff_flag, etc.) aredetermined, difference information and sign information of the lumafilter coefficients may be parsed/signaled (i.e., alf_luma_coeff_flag istrue), if indicated). Delta absolute value information(alf_luma_coeff_delata_abs syntax element) for each of the 12 filtercoefficients may be parsed/signaled. In addition, if the syntax elementalf_luma_coeff_delta_abs has a value, sign information (syntax elementalf_luma_coeff_delta_sign) may be parsed/signaled. The informationincluding the difference information and the sign information of theluma filter coefficients may be referred to as information about theluma filter coefficients.

The deltas of the filter coefficients may be determined along with thesign and stored. In this case, the deltas of the signed filtercoefficients may be stored in the form of an array, which may beexpressed as filterCoefficients. The deltas of the filter coefficientsmay be referred to as delta luma coefficients, and the deltas of thesigned filter coefficients may be referred to as signed delta lumacoefficients.

To determine the final filter coefficients from the signed delta lumacoefficients, the (luma) filter coefficients may be updated as thefollow equation.filterCoefficients[sigFiltIdx][j]+=filterCoefficients[sigFiltIdx][j]  [Equation2]

Here, j may indicate a filter coefficient index, and sigFiltIdx mayindicate a signaled filter index. j={0, . . . , 11} and sigFiltIdx={0, .. . alf_luma_filters_signaled_minus1}.

The coefficients may be copied into the final AlfCoeffL[filtIdx][j]. Inthis case, filtidx=0, . . . , 24 and j=0, . . . , 11.

The signed delta luma coefficients for a given filter index may be usedto determine the first 12 filter coefficients. For example, thethirteenth filter coefficient of the 7×7 filter may be determined basedon the following equation. The thirteenth filter coefficient mayindicate the above-described center tap filter coefficient.AlfCoeff_(L)[filtIdx][12]=128−Σ_(k)AlfCoeff_(C)[filtIdx][k]<<1  [Equation3]

Here, the filter coefficient index 12 may indicate a thirteenth filtercoefficient.

For example, in order to ensure bitstream conformance, the range of thevalues of the final filter coefficients AlfCoeffL[filtIdx][k] is 0, . .. , 11 may range from −2⁷ to 2⁷−1, and may range from 0 to 2⁸−1 when kis 12. Here, k may be replaced with j.

When processing on the luma component is performed, processing on thechroma component may be performed based on the syntax elementalf_chroma_idc. If the value of the syntax element alf_chroma_idc isgreater than 0, the minimum EG order information for the chromacomponent (i.e., the syntax element alf_chroma_min_eg_order_minus1) maybe parsed/signaled. According to the above-described embodiment of thepresent document, a 5×5 diamond filter shape may be used for the chromacomponent. In this case, the maximum golomb index may be 2. In thiscase, the EG order (or EG index) for the chroma component may bedetermined, for example, as follows.

TABLE 11 1. An initial constant, Kmin is set to bealf_chroma_min_eg_order_minus1 + 1 2. For each index, i in the range of{0 to maximum golomb index}, an intermediate array, i.e., KminTab[i] iscomputed, consisting of the sum of    KminTab[i] = Kmin +alf_chroma_eg_order_increase_flag[i] 3. Kmin is refreshed at the end ofeach iteration to be       Kmin = KminTab[i]

Based on the determination process, expGoOrderC may be derived asexpGoOrderC=KminTab. Through this, an array including EG orders can bederived, which can be used by the decoding apparatus. The expGoOrderCmay indicate the EG order (or EG index) for the chroma component.

If alf_luma_clip_flag is enabled (if available), the minimum exponentialGolomb order for clipping can be parsed. Similar to ALF coefficientsignaling, alf_luma_clip_eg_order_increase_flag may be parsed. Then, foreach filter index, alf_luma_doeff_flag is enabled (available), and iffilter coefficients (i.e., j=0 . . . 1) exist, the clipping index may beparsed.

Once the luma filter coefficients are reconstructed, the chromacoefficients may also be parsed. Furthermore, there may be a pre-definedgolomb order index (golombOrderIdxC). The pre-defined golomb order maybe used to determine a final golomb order for coding the coefficients.

For example, the pre-defined golomb order may be configured as, forexample, the following equation.golombOrderIdxC[ ]={0,0,1,0,0,1}  [Equation 4]

Here, the order k=expGoOrderC[golombOrderIdxC[j]], and j may representthe j-th signaled filter coefficient. For example, if j=2, it indicatesthe third filter coefficient, golomborderIdxY[2]=1, and thusk=expGoOrderC[1].

Based on this, absolute value information and sign information of thechroma filter coefficients may be parsed/signaled. Information includingabsolute value information and sign information of the chroma filtercoefficients may be referred to as information on the chroma filtercoefficients. For example, a 5×5 diamond filter shape may be applied tothe chroma component, and in this case, absolute delta information (thesyntax element alf_chroma_coeff_abs) for each of the six (chromacomponent) filter coefficients may be parsed/signaled. In addition, ifthe value of the syntax element alf_chroma_coeff_abs is greater than 0,sign information (the syntax element alf_chroma_coeff_sign) may beparsed/signaled. For example, the six chroma filter coefficients may bederived based on information on the chroma filter coefficients. In thiscase, the seventh chroma filter coefficient may be determined based onthe following equation, for example. The seventh filter coefficient mayrepresent the above-described center tap filter coefficient.golombOrderIdxC[ ]={0,0,1,0,0,1}  [Equation 5]

Here, the filter coefficient index 6 may indicate a seventh filtercoefficient. For reference, since the filter coefficient index startsfrom 0, a value of 6 may indicate a seventh filter coefficient.

For example, in order to ensure bitstream conformance, the range of thevalues of the final filter coefficients AlfCoeffC[filtIdx][k] may befrom −2⁷ to 2⁷−1 when k is 0, . . . , 5, and the range of the values ofthe final filter coefficients AlfCoeffC[filtIdx][k] may be from −2⁸ to2⁸−1 when k is 6. Here, k may be replaced with j.

Then, similarly to the case of luma, when a clipping index for chroma isdetermined, alf_luma_clip_flag may be checked. If the flag is enabled(if available), the minimum Golomb order for clipping may be parsed.Then the alf_chroma_clip_index may be parsed if the filter coefficientsare present.

As described above, when the (luma/chroma) filter coefficients arederived, ALF based filtering may be performed based on the filtercoefficients or a filter including the filter coefficients. Throughthis, modified reconstructed samples may be derived, as described above.In addition, multiple filters may be derived, and filter coefficients ofone of the multiple filters may be used for the ALF process. Forexample, one of the plurality of filters may be indicated based on thesignaled filter selection information. Or, for example, one of theplurality of filters may be selected based on the activity and/ordirectionality of the current block or the ALF target block, and filtercoefficients of the selected filter may be used for the ALF process.

Once the layer information from the APS and slice header is decoded, thecoding tree unit (CTU) is decoded. One Network Abstraction Layer (NAL)unit may include a slice header and slice data. Slice data mayfacilitate decoding of a coding tree unit.

The table below exemplarily represents the syntax of the coding treeunit.

TABLE 12 coding_tree_unit( ) { Descriptor  xCtb = ( CtbAddrInRs %PicWidthInCtbsY ) << CtbLog2SizeY  yCtb = ( CtbAddrInRs /PicWidthInCtbsY ) << CtbLog2SizeY  if( slice_sao_luma_flag | |slice_sao_chroma_flag )   sao( xCtb >> CtbLog2SizeY, yCtb >>CtbLog2SizeY )  if( slice_alf_enabled_flag )   alf_ctb_flag[ 0 ][xCtb >> Log2CtbSize ][yCtb >> Log2CtbSize ] ae(v)   if( alf_ctb_flag[ 0][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] ) {     if(num_alf_aps_ids > 0)      alf_ctb_use_first_aps_flag ae(v)     if(!alf_ctb_use_first_aps_flag ) {      if( num_alf_aps _ids > 1 )      alf_use_aps_flag ae(v)      if( alf_use_aps_flag )      alf_luma_prev_filter_idx_minus1 ae(v)      else      alf_luma_fixed_filter_idx ae(v)     }    }   if(slice_alf_chroma_idc = = 1 | | slice_alf_chroma_idc = = 3 )    alf_ctb_flag[ 1 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ]ae(v)   if( slice_alf_chroma_idc = = 2 | | slice_alf_chroma_idc = = 3 )    alf_ctb_flag[ 2 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ]ae(v)  }  if( slice_type = = I && qtbtt_dual_tree_intra_flag )  dual_tree_implicit_qt_split( xCtb, yCtb, CtbSizeY, 0 )  else  coding_tree( xCtb, yCtb, CtbSizeY, CtbSizeY, 1, 0, 0, 0, 0, 0, 0,SINGLE_TREE ) }

The semantics of syntax elements included in the syntax of Table 12 maybe expressed, for example, as shown in the following table.

TABLE 13   The CTU is the root node of the coding tree structure. Thearray IsInSmr[ x ][ y ] specifying whether the sample at ( x, y) islocated inside a shared merging candidate list region, is initialized asfollows for x = 0..CtbSizeY − 1 and y = 0..CtbSizeY − 1:     IsInSmr[ x][ y ] = FALSE alf_ctb_flag[ cIdx ][ xCtb >> Log2CtbSize ][ yCtb >>Log2CtbSize ] equal to 1 specifies that the adaptive loop filter isapplied to the coding tree block of the colour component indicated bycIdx of the coding tree unit at luma location ( xCtb, yCtb ),alf_ctb_flag [ cIdx ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ]equal to 0 specifies that the adaptive loop filter is not applied to thecoding tree, block of the colour component indicateD by cIdx of thecoding tree unit at luma location ( xCtb, yCtb ). When alf_ctb_flag[cIdx ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is not present, itis inferred to be equal to 0. alf_ctb_use_first_aps_flag equal to 1sepcifies that the filter information in APS withadaptive_parameter_set_id  equal  to  slice_alf_aps_id_luma[ 0 ]  is used. alf_ctb_use_first_aps_flag equal to 0 specifies that the luma CTBdoes not use the filter information in APS withadaptive_parameter_set_id equal to slice_alf_aps_id_luma[ 0 ]. Whenalf_ctb_use_first_aps_flag is not present, it is inferred to be equal to0. alf_use_aps_flag equal to 0 specifies that one of the fixedfilter-sets is applied to the luma CTB. alf_use_aps_flag equal to 1specifies that a filter set from an APS is applied to the luma CTB. Whenalf_use_aps_flag is not present, it is inferred to be equal to 0.alf_luma_prev_filter_idx_minus1 plus 1 specifies the previous filterthat is applied to the luma CTB. The value ofalf_luma_prev_filter_idx_minus1 shall be in a range of 0 tonum_alf_aps_ids − 2, inclusive. The variable AlfCtbFiltSetIdxY xCtb >>Log2CtbSize ][ yCtb >> Log2CtbSize ] specifying the filter set index forthe luma CTB at location ( xCtb, yCtb ) is derived as follows: − If     alf_ctb_use_first_aps     is     equal    to    1, AlfCtbFiltSetIdxY[ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is setequal to 16. −Otherwise,   if   alf_use_aps_flag   is   equal    to    0, AlfCtbFiltSetIdxY[ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is setequal to  alf_luma_fixed_filter_idx. − Otherwise, AlfCtbFiltSetIdxY[xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is set  equal to 17 +alf_luma_prev_filter_idx_minus1. alf_luma_fixed_filter_idx specifies thefixed filter that is applied to the hima CTB. The value ofalf_luma_fixed_filter_idx shall be in a range of 0 to 15, inclusive.

In CTU level processing, when ALF is enabled for the slice, the syntaxelement alf_ctb_flag may be parsed/signaled. alf_ctb_flag may specifythat ALF is applied to a coding tree block (CTB). When ALF is applied toCTB and the number of APS IDs transmitted is greater than 0, anotherflag alf_use_aps_flag may be parsed/signaled. When alf_use_aps_flag is1, it may specify that filter information of APS is applied to luma CTB.However, when alf_use_aps_flag is deactivated, one of the fixed filtersets may be applied to the CTB.

In an embodiment of the present document, a coding process forefficiently applying the above-described ALF is proposed. According toan embodiment of the present document, an example of a binarizationprocess for the syntax element alf_luma_fixed_filter_idx described aboveis proposed. Here, alf_luma_fixed_filter_idx may specify a fixed filterapplied to (luma) CTB (reconstructed samples).

In one example, if alf_use_aps_flag is true (value of 1), the filter setfrom APS may be applied to (luma) CTB (reconstructed samples) and basedon information about the previous filter (egalf_luma_prev_filter_idx_minus1) Filter coefficients may be derived.When alf_use_aps_flag is false (a value of 0), a fixed filter may beapplied to (luma) CTB (reconstructed samples). For example, the value ofalf_luma_fixed_filter_idx may be in the range of 0 to 15.

The table below shows an example of a binarization process of syntaxelements related to ALF.

TABLE 14 Syntax Binarization structure Syntax element Process Inputparameters coding alf_ctb_flag[ ][ ][ ] FL cMax = 1 treealf_ctb_use_first_aps_flag FL cMax = 1 unit( ) alf_use_aps_flag FL cMax= 1 alf_luma_fixed_filter_idx TB cMax = 15 alf luma prev filter idx TBcMax = num_alf_ minus1 aps_ids − 2

In Table 14, the syntax element alf_luma_fixed_filter_idx may be codedbased on a truncated binary (TB) binarization process. For example, themaximum value (ex. cMax) related to the syntax element may be 15.According to the TB binarization process, only complexity can beunnecessarily increased without any additional benefit. To this end, thefollowing example based on a fixed length (FL) binarization process isproposed.

The table below shows another example of information for binarization ofsyntax elements related to the ALF procedure.

TABLE 15 Syntax Binarization structure Syntax element Process Inputparameters coding alf_ctb_flag[ ][ ][ ] FL cMax = 1 treealf_ctb_use_first_aps_flag FL cMax = 1 unit( ) alf_use_aps_flag FL cMax= 1 alf_luma_fixed_filter_idx FL cMax = 15 alf luma prev filter idx TBcMax = num_alf_ minus1 aps_ids − 2

Table 15 is described with a focus on differences from Table 14. InTable 15, the syntax element alf_luma_fixed_filter_idx may be codedbased on the FL binarization process. For example, the maximum value(ex. cMax) related to the syntax element may be expressed using 4 bits.

The table below shows an example of assigning ctxInc to syntax elementswith context coded bins as context coded bins according to Table 15above. In Table 15, for example, a bin having a binIdx greater than orequal to 5 may not exist (or may not be available) for the syntaxelement alf_luma_fixed_filter_idx.

TABLE 16 binIdx Syntax element 0 1 2 3 4 >= 5 end_of_brick_ one_bitterminate na na na na na alf_ctb_ 0 . . . 8 na na na na na flag[ ][ ][ ](clause 9.5.4.2.2) alf_ctb_use_first_ 0 na na na na na aps_flagalf_use_aps_flag 0 na na na na na alf_luma_fixed_ bypass bypass bypassbypass bypass na filter_idx alf_luma_ prev_filter_ bypass bypass bypassbypass bypass by- pass idx_minus1

According to an embodiment of the present disclosure, an example of abinarization procedure for the aforementioned syntax elementalf_luma_fixed_filter_set_idx is proposed. Here,alf_luma_fixed_filter_set_idx may specify an index of a fixed filterset.

For example, a fixed length (FL) binarization procedure may be appliedfor signaling of alf_luma_fixed_filter_set_idx.

The table below exemplifies the ALF data syntax.

TABLE 17 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1) if( alf_luma_filter_signal_flag ) {   alf_lama_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAlfFilters; filtIdx++ )     alf_luma_coeff_delta_idx[filtIdx ] tb(v)   }   alf_luma_use_fixed_filter_flag u(1)   if(alf_luma_use_fixed_filter_flag ) {    alf_luma_fixed_filter_set_idx u(4)   alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag ) {     for( i = 0; i <NumAlfFilters; i++ )      alf_luma_fixed_filter_pred_flag[ i ] u(1)    } }

Referring to the above table, alf_luma_fixed_filter_set_idx may be codedbased on a fixed length (FL) binarization procedure rather than atruncated binary (TB) binarization procedure. The semantics ofalf_luma_fixed_filter_set_idx included in the table may be expressed,for example, as shown in the following table.

TABLE 18 alf_luma_fixed_filler_set_idx specifies a fixed filler setindex. The value of alf_luma_fixed_filter_set_idx shall be in range of 0to 15, inclusive.

In an embodiment of the present document, a signaling method related toALF coefficients for simplifying a signaling derivation process ofGolomb orders is proposed. For example, in an embodiment of the presentdocument, the proposed method may be achieved through fixed signaling.In one example, the kth order of the exponential Golomb code may befixed at 2 for luma and 3 for chroma. However, the embodiment of thepresent document is not necessarily limited by such an example, and thek order may be fixed to another appropriate value.

The table below shows the syntax of the ALF data field to which theabove-described simplification is applied.

TABLE 19 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1) if( alf_luma_filter_signal_flag ) {   alf_luma_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAlfFilter; filtIdx++ )     alf_luma_coeff_delta_idx[filtIdx ] tb(v)   }   alf_luma_use_fixed_filter_flag u(1)   if(alf_luma_use_fixed_filter_flag ) {    alf_luma_fixed_filter_set_idxtb(v)    alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag ) {     for( i = 0; i <NumAlfFilters: i++ )      alf_luma_fixed_filter_pred_flag[ i ]    }   }  alf_luma_coeff_delta_flag u(1)   if( !alf_luma_coeff_delta_flag &&alf_luma_num_filters_signalled_minus1 > 0 )   alf_luma_coeff_delta_prediction_flag u(1)   if(alf_luma_coeff_delta_flag ) {    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ )    alf_luma_coeff_flag[ sfIdx ]   }   for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {    if(alf_luma_coeff_flag[ sfIdx ] ) {     for (j = 0; j < 12; j++ ) {     alf_luma_coeff_delta_abs[ sfIdx ][ j ] uek(v)      if(alf_luma_coeff_delta_abs[ sfIdx ][ j ] )      alf_luma_coeff_delta_sign[ sfIdx ][ j ] u(1)     }    }   }   if(alf_luma_clip_flag ) {    alf_luma_clip_min_eg_order_minus1 ue(v)   for( i = 0; i < 3; i++ )     alf_luma_clip_eg_order_increase_flag[ i] u(1)    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     if(alf_luma_coeff_flag[ sfIdx ] )      for( j = 0; j < 12; j++ ) {      if( filtCoeff[ sfIdx ][ j ] )        alf_luma_clip_idx[ sfIdx ][ j] uek(v)      }     }    }   }  }  if(alf_chroma_filter_signal_flag ) {  alf_chroma_clip_flag u(1)   for( j = 0; j < 6; j++ ) {   alf_chroma_coeff_abs[ j ] uek(v)    if( alf_chroma_coeff_abs[ j ] > 0)     alf_chroma_coeff_sign[ j ] u(1)   }   if( alf_chroma_clip_flag ) {   alf_chroma_clip_min_eg_order_minus1 ue(v)    for( i = 0; i < 2; i++ )    alf_chroma_clip_eg_order_increase_flag[ i ] u(1)    for( j = 0; j <6; j++ ) {     if( alf_chroma_coeff_abs[ j ] )      alf_chroma_clip_idx[j ] uek(v)    }   }  } }

Semantics of syntax elements included in the syntax of Table 19 may beexpressed, for example, as shown in the following table.

TABLE 20   alf_luma_coeff_flag[ sigFiltIdx ] equal 1 specifies that thecoefficients of the luma filter indicated by sigFiltIdx are signalled,alf_luma_coef_flag[ sigFiltIdx ] equal to 0 specifies that all filtercoefficients of the luma filter indicated by sigFiltIdx are set equal to0. When not present, alf_luma_coeff_flag[ sigFiltIdx ] is set equalto 1. alf_luma_coeff_delta_abs[ sigFiltIdx ][ j ] specifies the absolutevalue of the j-th coefficient delta  of  the  signalled  luma  filter indicated  by  sigFiltIdx.  When alf_luma_coeff_delta_abs[ sigFiltIdx][ j ] is not present, it is inferred to be equal 0. The order k of theexp-Golomb binarization uek(v) is    k = 2 alf_luma_coeff_delta_sign[sigFiltIdx ][ j ] specifies the sign of the j-th luma coefficient of thefilter indicated by sigFiltIdx as follows: − Ifalf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is equal to 0, thecorresponding luma filter  coefficient has a positive value. − Otherwise(alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is equal to 1), thecorresponding  luma filter coefficient has a negative value. Whenalf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is not present, it isinferred to be equal to 0. The      variable      filterCoefficients[sigFiltIdx ][ j ]      with sigFiltIdx =0..alf_luma_num_filters_signalled_minus1, j = 0..11 is initialized asfollows:   filterCoefficients[ sigFiltIdx ][ i ] =alf_luma_coeff_delta_abs[ sigFiltIdx ][ j ]   *               (1 − 2 *alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] ) Whenalf_luma_coeff_delta_prediction_flag is equal 1, filterCoefficients[sigFiltIdx ][ j ] with sigFiltIdx =1..alf_luma_num_filters_signalled_minus1 and j = 0..11 are modified asfollows:   filterCoefficients[ sigFiltIdx ][ j ] +− filterCoefficients[sigFilfIdx − 1 ][ j ] The luma filter coefficients AlfCoeff_(L) withelements AlfCoeff_(L)[ filtIdx ][ j ], with filtIdx = 0..NumAlfFilters −1 and j = 0..11 are derived as follows   AlfCoeff_(L)[ filtIdx ][ j ] =filterCoefficients[ alf_luma_coeff_delta_idx[ filtIdx ] ][ j ] The lastfilter coefficients AlfCoeff_(L)[ filtIdx ][ 12 ] for filtIdx =0..NumAlfFilters − 1 are derived as follows:   AlfCoeff_(L)[ filtIdx ][12 ] = 128 Σ_(k) ( AlfCoeff_(L)[ filtIdx ][ k ] << 1 ), with k = 0..11It is a requirement of bitstream conformance that the values ofAlfCoef_(L)[ filtIdx ][ j ] with filtIdx = 0..NumAlfFilters − 1, j =0..11 shall be in the range of −2⁷ to 2⁷ − 1, inclusive and that thevalues of AlfCoef_(L)[ filtIdx ][ 12 ] shall be in the range of 0 to 2⁸− 1, inclusive. alf_chroma_coeff_abs[ j ] specifies the absolute valueof the j-th chroma filter coefficient. When alf_chroma_coeff_abs[ j ] isnot present, it is inferred to be equal 0. It is a requirement ofbitstream conformance that the values of alf_chroma_coeff_abs[ j ] shallbe in the range of 0 to 2⁷ − 1, inclusive. The order k of the exp-Golombbinarization uek(v) is derived as follows:   k = 3

In an embodiment of the present document, a method of usingpre-determined Golomb orders for each ALF filter coefficient isproposed.

The table below exemplarily shows the Golomb order (kth order) used fordecoding the ALF luma coefficients. C0 to C11 denote ALF lumacoefficient indices (or, for example, filter coefficients depending onpositions in the filter shape for the ALF according to FIG. 6 ), where,for example, the kth order may be 2 or 3.

TABLE 21 Coefficient k^(th) Index order C₀ 2 C₁ 2 C₂ 2 C₃ 2 C₄ 2 C₅ 3 C₆3 C₇ 3 C₈ 2 C₉ 2 C₁₀ 2 C₁₁ 3

The table below exemplifies semantics based on coding according to thisembodiment. For example, the table below may be semantics modified tofix the Golomb order for the luma coefficients or the chromacoefficients.

TABLE 22   The variable NumAlfFilters specify mg the number ot differentadaptive loop filters is set equal to 25. alf_luma_num_filters_signalledminus1 plus 1 specifies the number of adpative loop filter classes forwhich luma coefficients can be signaled. The value ofalf_luma_num_filters_ signalled_minus1 shall be in the range of 0 toNumAlfFilters − 1, inclusive. The maximum value maxVal of the truncatedbinary binarization tb(v) is set equal to NumAlfFilters − 1.alf_luma_coeff_delta_idx[ filtIdx ] specifies the indices of thesignalled adaptive loop filter luma coefficient deltas for the filterclass indicated by filtIdx ranging from 0 to NumAlfFilters − 1. Whenalf_luma_coeff_delta_idx[ filtIdx ] is not present, it is inferred to beequal to 0. The maximum value maxVal of the truncated binarybinarization th(v) is set equal toalf_luma_num_filters_signalled_minus1. alf_luma_coeff_delta_flag equalto 1 indicates that alf_luma_coeff_delta_prediction_flag is not  signalled.   alf_luma_coeff_delta_flag   equal   to  0   indicates  that alf_luma_coeff_delta_prediction_flag may be signalled.alf_luma_coeff_delta_prediction_flag equal to 1 specifies that thesignalled luma filter coefficient deltas are predicted from the deltasof the previous luma coefficients. alf_luma_coeff_delta_prediction_flagequal to 0 specifies that the signalled luma filter coefficient deltasare not predicted from the deltas of the previous luma coefficients.When not present, alf_luma_coeff_delta_prediction_flag is inferred to beequal to 0. alf_luma_coeff_flag[ sigFiltIdx ] equal 1 specifies that thecoefficients of the luma filter indicated by sigFiltIdx are signalled.alf_luma_coeff_flag[ sigFiltIdx ] equal to 0 specifies that all filtercoefficients of the luma filter indicated by sigFiltIdx are set equal to6. When not present, alf_luma_coeff_flag[ sigFiltIdx ] is set equalto 1. alf_luma_coeff_delta_abs[ sigFiltIdx ][ j ] specifies the absolutevalue of the j-th coefficient delta   of   the   signalled   luma  filter   indicated   by   sigFiltIdx.   When alf_luma_coeff_delta_abs[sigFiltIdx ][ j ] is not present, it is inferred to be equal 0. Theorder k of the exp-Golomb binarization uek(v) is     expGoOrderY = { 2,2, 2, 2, 2, 3, 3, 3, 2, 2, 2, 3 }     k = expGoOrderY[ j ]alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] specifies the sign of thej-th luma coefficient of the filter indicated by sigFiltIdx as follows:− If alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is equal to 0, thecorresponding luma filter  coefficient has a positive value. − Otherwise(alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is equal to 1), thecorresponding  luma filter coefficient has a negative value. Whenalf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] is not present, it isinferred to be equal to 0. The      variable      filterCoefficients[sigFiltIdx ][ j ]       with sigFiltIdx =0..alf_luma_num_filters_sigralled_minus1, j = 0.11 is initialized asfollows:   filterCoefficients[ sigFiltIdx ][ j ] =alf_luma_coeff_delta_abs[ sigFiltIdx ][ j ]    *               ( 1 − 2 *alf_luma_coeff_delta_sign[ sigFiltIdx ][ j ] ) Whenalf_luma_coeff_delta_prediction_flag is equal 1, filterCoefficients[sigFiltIdx ][ j ] with sigFiltIdx =1..alf_luma_num_filters_signalled_minus1 and j = 0.11 are modified asfollows:   filterCoefficients[ sigFiltIdx ][ j ] += filterCoefficients[sigFiltdx − l ][ j ] The  luma  filter  coefficients  AlfCoeff_(L)  with elements  AlfCoeff_(L)[ filtIdx ][ j ],  with filtIdx =0..NumAlfFilters − 1 and j = 0..11 are derived as follows  AlfCoeff_(L)[ filtIdx ][ j ] = filterCoefficients[alf_luma_coeff_delta_idx[ filtIdx ] ][ j ] The last filter coefficientsAlfCoeff_(L)[ filtIdx ][ 12 ] for filtIdx = 0..NumAlfFilters − 1 arederived as follows:   AlfCoeff_(L)[ filtIdx ][ 12 ] = 128 − Σ_(k) (AlfCoeff_(L)[filtIdx][ k ] << 1 ), with k = 0..11 It is a requirement ofbitstream conformance that the values of AlfCoeff_(L)[ filtIdx ][ j ]with filtIdx = 0..NumAlfFilters − 1, j = 0..11 shall be in the range of−2⁷ to 2⁷ − 1, inclusive and that the values of AlfCoeff_(L)[ filtIdx ][12 ] shall be in the range of 0 to 2⁸ − 1, inclusive.alf_chroma_coeff_abs[ j ] specifies the absolute value of the j-thchroma filter coefficient. When alf_chroma_coeff_abs[ j ] is notpresent, it is inferred to be equal 0. It is a requirement of bitstreamconformance that the values of alf_chroma_coeff_abs[ j ] shall be in therange of 0 to 2⁷ − 1, inclusive. The order k of the exp-Golombbinarization uek(v) is   k = 3 alf_chroma_coeff_sign[ j ] specifies thesign of the j-th chroma filter coefficient as follows: − Ifalf_chroma_coeft_sign[ j ] is equal to 0, the corresponding chromafilter coefficient has a  positive value. − Otherwise(alf_chroma_coeff_sign[ j ] is equal to 1), the corresponding chromafilter  coefficient has a negative value. When alf_chroma_coeff_sign[ j] is not present, it is inferred to be equal to 0. The chroma filtercoefficients AlfCoeff_(C) with elements c_(C)[ j ], with j = 0..5 arederived as follows:   AlfCoeff_(C)[ j ] = alf_chroma_coeff_abs[ j ] * (1 − 2 * alf_chroma_coeff_sign[ j ] ) The last filter coefficient for j =6 is derived as follows:   AlfCoeff_(C)[ 6 ] = 128 − Σ_(k)(AlfCoeff_(C)[ k ] << 1 ), with k = 0..5 It is a requirement ofbitstream conformance that the values of AlfCoeff_(C)[ j ] with j = 0..5shall be in the range of −2⁷ − 1 to 2⁷ − 1, inclusive and that thevalues of AlfCoeff_(C)[ 6 ] shall be in the range of 0 to 2⁸ − 1,inclusive.

In an embodiment of the present document, a method of signaling a fixedGolomb order only for chroma components is proposed. That is, an exampleof signaling for fixing the Golomb order for chroma components isimplemented according to this embodiment.

The table below exemplarily shows the syntax of the ALF data field basedon coding according to an embodiment of the present document.

TABLE 23 alf_data( adaptation_paramerer_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1) if( alf_luma_filter_signal_flag ) {   alf_luma_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAlfFilters; filtIdx++ )     alf_luma_coeff_delta_idx[filtIdx ] tb(v)   }   alf_luma_use_fixed_filter_flag u(1)   if(alf_luma_num_fixed_filter_flag ) {    alf_luma_fixed_filter_set_idxtb(v)    alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag ) {     for( i = 0; i <NumAlfFilters; i++ )       alf_luma_fixed_filter_pred_flag[ i ] u(1)   }   }   alf_luma_coeff_delta_flag u(1)   if(!alf_luma_coeff_delta_flag && alf_luma_num_filters_signalled_minus1 > 0)    alf_luma_coeff_delta_prediction_flag u(1)  alf_luma_min_eg_order_minus1 ue(v)   for( i = 0; i < 3; i++ )   alf_luma_eg_order_increase_flag[ i ] u(1)   if(alf_luma_coeff_delta_flag ) {    for( sfIdx = 0; sfIdx =alf_luma_num_filters_signalled_minus1; sfIdx++)     alf_luma_coeff_flag[sfIdx ] u(1)   }   for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {    if(alf_luma_coeff_flag[ sfIdx ] ) {     for( j = 0; j < 12; j++ )     alf_luma_coeff_delta_abs[ sfIdx ][ j ] uek(v)      if(alf_luma_coeff_delta_abs[ sfIdx ][ j ] )      alf_luma_coeff_delta_sign[ sfIdx ][ j ] u(1)     }    }   }   if(alf_luma_clip _flag ) {    alf_luma_clip_min_eg_order_minus1 ue(v)   for( i = 0; i < 3; i++ )     alf_luma_clip_eg_order_increase_flag[ i] u(1)    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     if(alf_luma_coeff_flag[ sfIdx ] ) {      for ( j = 0: j < 12; j++ )      if( filtCoeff[ sfIdx ][ j ] )        alf_luma_clip_idx[ sfIdx ][ j] uek(v)      }     }    }   }  }  if( alf_chroma_filter_signal_flag ) {  alf_chroma_clip_flag u(1)   for( j = 0; j < 6; j++ ){   alf_chroma_coeff_abs[ j ] uek(v)    if( alf_chroma_coeff_abs[ j ] > 0)     alf_chroma_coeff_sign[ j ] u(1)   }   if( alf_chroma_clip_flag ) {   alf_chroma_clip_min_eg_order_minus1 ue(v)    for( i = 0; i < 2; i++ )    alf_chroma_clip_eg_order_increase_flag[ i ] u(1)    for( j = 0; j <6; j++ ){     if( alf_chroma_coeff_abs[ j ] )      alf_chroma_clip_idx[j ] uek(v)    }   }  } }

Semantics of syntax elements included in the syntax of Table 23 may beexpressed, for example, as shown in the following table.

TABLE 24   alf_chroma_coeff_abs[ j ] specifies the absolute value of thej-th chroma filter coefficient. When alf_chroma_coeff_abs[ j ] is notpresent, it is inferred to be equal 0. It is a requirement of bitstreamconformance that the values of alf_chroma_coeff_abs[ j ] shall be in therange of 0 to 2⁷ − 1, inclusive. The order k of the exp-Golombbinarization uek(v) is:        k = 3

In an embodiment of the present document, fixed signaling for codinginformation about ALF clipping (eg, alf_luma_clip_idx[sfIdx][j],alf_chroma_clip_idx[j] etc.) is proposed. For example, the k orderGolomb may be pre-determined or pre-fixed and used as in the embodimentsdescribed above.

The table below exemplifies the syntax of the ALF data field based oncoding according to the present embodiment.

TABLE 25 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1) if( alf_luma_filter_signal_flag ) {   alf_luma_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAlfFilters; filtIdx++ )     alf_luma_coef_delta_idx[filtIdx ] tb(v)   }   alf_luma_use_fixed_filter_flag u(1)   if(alf_luma_use_fixed_filter_flag ) {    alf_luma_fixed_filter_set_idxtb(v)    alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag {     for( i = 0; i <NumAlfFilters; i++ )      alf_luma_fixed_filter_pred_flag[ i ] u(1)    }  }   alf_luma_coeff_delta_flag u(1)   if( !alf_luma_coeff_delta_flag &&alf_luma_num_filters_signalled_minus1 > 0 )   alf_luma_coeff_delta_prediction_flag u(1)  alf_luma_min_eg_order_minus1 ue(v)   for( i = 0; i < 3; i++ )   alf_luma_eg_order_increase_flag[ i ] u(1)   if(alf_luma_coeff_delta_flag ) {    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ )    alf_luma_coeff_flag[ sfIdx ] u(1)   }   for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {    if(alf_luma_coeff_flag[ sfIdx ] ) {     for( j = 0; j < 12; j++ ) {     alf_luma_coeff_delta_abs[ sfIdx ][ j ] uek(v)      if(alf_luma_coeff_delta_abs[ sfIdx ][ j ] )      alf_luma_coeff_delta_sign[ sfIdx ][ j ] u(1)     }    }   }   if(alf_luma_clip_flag ) {    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     if(alf_luma_coeff_flag[ sfIdx ][ j ] {      for( j = 0; j < 12; j++ ) {      if( filtCoeff[ sfIdx ][ j ] )        alf_luma_clip_idx[ sfIdx ][ j] uek(v)      }     }    }   }  }  if( alf_chroma_filter_signal_flag ) {  alf_chroma_clip_flag u(1)   alf_chroma_min_eg_order_minus1 ue(v)  for( i = 0; i < 2; i++ )    alf_chroma_eg_order_increase_flag[ i ]u(1)   for( j = 0; j < 6; j++ ) {    alf_chroma_coeff_abs[ j ] uek(v)   if( alf_chroma_coeff_abs[ j ] > 0 )     alf_chroma_coeff_sign[ j ]u(1)   }   if( alf_chroma_clip_flag ) {    for( j = 0; j < 6; j++ ) {    if( alf_chroma_coef_abs[ j ] )      alf_chroma_clip_idx[ i ] uek(v)   }   }  } }

Semantics of syntax elements included in the syntax of Table 25 may beexpressed, for example, as shown in the following table.

TABLE 26   alf_luma_clip_idx[ sfIdx ][ j ] specifies the clipping indexof the clipping value to use before multiplying by the j-th coefficientof the signalled luma filter indicated by sfId x. Whenalf_luma_clip_idx[ sfIdx ][ j ] is not present, it is inferred to beequal 0 (no clipping): It is a requirement of bitstream conformance thatthe values of alf_luma_clip_ idx[ sfIdx ][ j ] with sfIdx =0..alf_luma_num_filters_signalled_minus1 and j = 0..11 shall be in therange of 0 to 3, inclusive. The order k of the exp-Golomb binarizationuek(v) is:   k = 2 The variable filterClips[ sfIdx ][ j ] with sfIdx =0..alf_luma_num_filters_signalled_minus1,  j = 0..11 is initialized asfellows:   filterClips[ sfIdx ][ j ] = Round(2^(( BitDepthY * ( 4 − alf)_luma_clip_idx[ sfIdx ][ j ] ) / 4) ) Theluma filter clipping values AlfClip_(L)[ adaptation_parameter_set_id ]with elements AlfClip_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j], with filtIdx = 0..NumAlfFilters − 1 and j = 0.11 are derived asfollows:   AlfClip_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ] =filterClips[ alf_luma_coeff_delta_   idx[ filtIdx ] ][ j ]alf_chroma_clip_flag equal to 0 specifies that linear adaptive loopfiltering is applied on chroma components; alf_chroma_clip_flag equal to1 specifies that non-linear adaptive loop filtering is applied on chromacomponent. When not present, alf_chroma_clip_flag is inferred to beequal to 0. alf_chroma_min_eg_order_minus1 plus 1 specifies the minimumorder of the exp-Golomb code for chroma filter coefficient signalling.The value of alf_chroma_min_eg_order_minus1 shall be in the range of 0to 6, inclusive. alf_chroma_eg_order_increase_flag[ i ] equal to 1specifies that the minimum order of the exp-Golomb code for chromafilter coefficient signalling is incremented by 1.alf_chroma_eg_order_increase_flag[ i ] equal to 0 specifies that theminimum order of the exp- Golomb code for chroma filter coefficientsignalling is not incremented by 1 The order expGoOrderC[ i ] of theexp-Golomb code used to decode the values of alf_chroma_coeff_abs[ j ]is derived as follows:   expGoOrderC[ i ] =alf_chroma_min_eg_order_minus1 + 1 + alf_chroma_eg_order_  increase_flag[ i ] alf_chroma_coeff_abs[ j ] specifies the absolutevalue of the j-th chroma filter coefficient. When alf_chroma_coeff_abs[j ] is not present, it is inferred to be equal 0. It is a requirement ofbitstream conformance that the values of alf_chroma_coeff_abs[ j ] shallbe in the range of 0 to 2⁷ − 1, inclusive. The order k of the exp-Golombbinarization uek(v) is derived as follows:   golombOrderIdxC[ ] = { 0,0, 1,0, 0, 1 }   k = expGoOrderC[ golombOrderIdxC[ j ] ]alf_chroma_coeff_sign[ j ] specifies the sign of the j-th chroma filtercoefficient as follows: − If alf_chroma_coeff_sign[ i ] is equal to 0,the corresponding chroma filter coefficient has a  positive value. −Otherwise (alf_chroma_coeff_sign[ j ] is equal to 1), the correspondingchroma filter  coefficient has a negative value.When_alf_chroma_coeff_sign[ j ] is not present, it is inferred to beequal to 0. The chroma filter coefficients AlfCoeff_(C)[adaptation_parameter_set_id ] with elements AlfCoeff_(C)[adaptation_parameter_set id ][ j ], with j = 0..5 we derived as follows: AlfCoeff_(C)[ adaptation_parameter set_id ][ j ] =alf_chroma_coeff_abs[ j ] *                  ( 1 − 2 *alf_chroma_coeff_sign[ j ] ) It  is  a  requirement  of  bitstream conformance  that  the  values  of AlfCoeff_(C)[adaptation_parameter_set_id ][ j ] with j = 0..5 shall be in the rangeof −2⁷ − 1 to 2⁷ − 1, inclusive. alf_chroma_clip_idx[ j ] specifies theclipping index of the clipping value to use before multiplying by thej-th coefficient of the chroma filter. When alf_chroma_clip_idx[ j ]  isnot present, it is inferred to be equal 0 (no clipping), it is arequirement of bitstream conformance that the values ofalf_chroma_clip_idx[ j ] with j = 0..5 shall be in the range of 0 to 3,inclusive. The order k of the exp-Golomb binarization uek(v) is derivedas follows:   k = 3 The chroma filter clipping values AlfClip_(C)[adaptation_parameter_set_id ] with elements AlfClip_(C)[adaptation_parameter_set_id ][ j ], with j = 0..5 are derived asfollows: AlfClip_(C)[ adaptation_parameter_set_id ][ j ] = Round(2^(( BitDepthC − 8 ) *) 2^(( 8 * ( 3 − alf)_chroma_clip_idx[ j ] ) / 3 ))

In an embodiment of the present document, a signaling combination fromvarious viewpoints for the ALF data field based on (or relying on)exponential Golomb coding is proposed. That is, an example of signalingcombining at least one of the above-described embodiments may beimplemented through this embodiment.

The table below exemplarily shows the syntax of the ALF data field basedon coding according to the present embodiment.

TABLE 27 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag  alf_chroma_filter_signal_flag u(1)  if(alf_luma_filter_signal_flag ) (   alf_luma_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAlfFilters; filtIdx++ )     alf_luma_coeff_delta_idx[filtIdx ] tb(v)   }   alf_ luma_use_fixed_filter_flag u(1)   if(alf_luma_use_fixed_filter_flag ) {    alf_luma_lixed_filter_set_idxtb(v)    alf_luma_fixed_filter_pred_present_flag u(1)    if(alf_luma_fixed_filter_pred_present_flag ) {     for( i = 0; i <NumAlfFilters: i++ )      alf_luma_fixed_filter_pred_flag[ i ] u(1)    }  }   alf_luma_coeff_delta_flag u(1)  if( !alf_luma_coeff_delta_flag &&alf_luma_num_filters_signalled_minus1 > 0 )  alf_luma_coeff_delta_prediction_flag u(1)  if(alf_luma_coeff_delta_flag ) {   for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ )    alf_luma_coeff_flag[sfIdx ] u(1)  }  for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ){   if(alf_luma_coeff_flag[ sfIdx ] ) {    for( j = 0; j < 12; j++ ) { uek(v)    alf_luma_coeff_delta_abs[ sfIdx ][ j ]     if(alf_luma_coeff_delta_abs[ sfIdx ][ j ] )      alf_luma_coeff_delta_sign[sfIdx ][ j ] u(1)    }   }  }  if( alf_luma_clip_flag ) {    for( sfIdx= 0; sfIdx <= alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     if(alf_luma_coeff_flag[ sfIdx ] ) {      for( j = 0; j < 12; j++ ) {      if( filtCoeff[ sfIdx ][ j ] )        alf_luma_clip_idx[ sfIdx ][ j] uek(v)      }     }    }   }  }  if( alf_chroma_filter_signal_flag ) {  alf_chroma_clip_flag u(1)   for( j = 0; j < 6; j++ ) {   alf_chroma_coeff_abs[ j ] uek(v)    if( alf_chroma_coeff_abs[ j ] > 0)     alf_chroma_coeff_sign[ j ] u(1)   }   if( alf_chroma_clip_flag ) {   for( j = 0; j < 6; j++ ) {     if( alf_chroma_coeff_abs[ j ] )     alf_chroma_clip_idx[ j ] uek(v)    }   }  } }

The semantics of syntax elements included in the syntax of Table 27 maybe expressed, for example, as shown in the following table.

TABLE 28   alf_luma_coeff_flag[ sfIdx ] equal 1 specifies that thecoefficients of the luma filter indicated by sfIdx are signalled.alf_luma_coeff_flag[ sfIdx ] equal to 0 specifies that all filtercoefficients of  the  luma  filter  indicated  by  sfIdx  are  set equal  to  0.  When  not  present, alf_luma_coeff_flag[ sfIdx ] is setequal to 1. alf_luma_coeff_delta_abs[ sfIdx ][ j ] specifies theabsolute value of the j-th coefficient delta of the signalled lumafilter indicated by sfIdx. When alf_luma_coeff_delta_abs[ sfIdx ][ j ]is not present, it is inferred to be equal 0. The order k of theexp-Golomb binarization uek(v) is:    k = 2 alf_luma_clip_idx[ sfIdx ][j ] specifies the clipping index of the clipping value to use beforemultiplying by the j-th coefficient of the signalled luma filterindicated by sfId x. When alf_luma_clip_idx[ sfIdx ][ j ] is notpresent, it is inferred to be equal 0 ( no clipping). It is arequirement of bitstream conformance that the values of alf_luma_clip_idx[ sfIdx ][ j ] with sfIdx = 0..alf_luma_num_filters_signalled_minus1and j = 0..11 shall be in the range of 0 to 3, inclusive. The order k ofthe exp-Golomb binarization uek(v) is:    k = 2 The variablefilterClips[ sfIdx ][ j ] with sfIdx =0..alf_luma_num_filters_signalled_minus1,  j = 0.11 is initialized asfollows:  filterClips[ sfIdx ][ j ] = Round(2^(( BitDepthY * ( 4 − alf)_luma_clip_idx[sfIdx][ j ] ) / 4 ) ) The lumafilter clipping values AlfClip_(L)[ adaptation_parameter_set_id ] withelements AlfClip_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ],with filtIdx = 0..NumAlfFilters − 1 and j = 0.11 are derived as follows: AlfClip_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ] =filterClips[ alf_luma_coeff_delta_  idx[ filtIdx ][ j ]alf_chroma_clip_flag equal to 0 specifies that linear adaptive loopfiltering is applied on chroma components: alf_chroma_clip_flag equal to1 specifies that non-linear adaptive loop filtering is applied on chromacomponent. When not present, alf_chroma_clip_flag is inferred to beequal to 0. alf_chroma_coeff_abs[ j ] specifies the absolute value ofthe j-th chroma filter coefficient. When alf_chroma_coeff_abs[ j ] isnot present, it is inferred to be equal 0. It is a requirement ofbitstream conformance that the values of alf_chroma_coeff_abs[ j ] shallbe in the range of 0 to 2⁷ − 1, inclusive. The order k of the exp-Golombbinarization uek(v) is:    k = 3 alf_chroma_coeff_sign[ j ] specifiesthe sign of the j-th chroma filter coefficient as follows: − Ifalf_chroma_coeff_sign[ j ] is equal to 0, the corresponding chromafilter coefficient has a  positive value. − Otherwise(alf_chroma_coeff_sign[ j ] is equal to 1), the corresponding chromafilter  coefficient has a negative value. When alf_chroma_coeff_sign[ j] is not present, it is inferred to be equal to 0. The chroma filtercoefficients AlfCoeff_(C)[ adaptation_parameter_set_id ] with elements.AlfCoeff_(C)[ adaptation_parameter_set_id ][ j ], with j = 0..5 arederived as follows:  AlfCoeff_(C)[ adaptation_parameter_set_id ][ j ] =alf_chroma_coeff_abs[ j ] *                  ( 1 − 2 *alf_chroma_coeff_sign[ j ] ) It  is  a  requirement  of  bitstream conformance  that  the  values  of AlfCoeff_(C)[adaptation_parameter_set_id ][ j ] with j = 0..5 shall be in the rangeof −2⁷ − 1 to 2⁷ − 1, inclusive. alf_chroma_clip_idx[ i ] specifies theclipping index of the clipping value to use before multiplying by thej-th coefficient of the chroma filter. When alf_chroma_clip_idx[ j ]  isnot present, it is inferred to be equal 0 (no clipping). It is arequirement of bitstream conformance that the values ofalf_chroma_clip_idx[ j ] with j = 0..5 shall be in the range of 0 to 3,inclusive. The order k of the exp-Golomb binarization uek(v) is:    k =3 The chroma filter clipping values AlfClip_(C)[adaptation_parameter_set_id ] with elements AlfClip_(C)[adaptation_parameter_set_id ][ j ], with j = 0..5 are derived asfollows: AlfClip_(C)[ adaptation_parameter_set_id ][ j ] = Round(2^(( BitDepthC − 8 ) *) 2^(( 8 * ( 3 − alf)_chroma_clip_idx[ j ] ) / 3 ))

In an embodiment of the present disclosure, a method for preventing APSID for chroma from being signaled and inferred is proposed.

The table below exemplifies the syntax of the slice header based oncoding according to the present embodiment.

TABLE 29 slice_header( ) { Descriptor  slice_pic_parameter_set_id ue(v). . .  if( sps_alf_enabled_flag ) {   slice_alf_enabled_flag u(1)   if(slice_alf_enabled_flag ) {    slice_num_alf_aps_ids_luma tb(v)    for( i= 0; i < slice_num_alf_aps_ids_luma; i++ )     slice_alf aps_id_luma[ i] u(5)    slice_alf_chroma_idc n(v)   if( slice_alf_chroma_idc)    slice_alf_aps_id_chroma u(5) . . .

The semantics of syntax elements included in the syntax of the tableabove may be expressed, for example, as shown in the following table.

TABLE 30 slice_num_alf_aps_ids_luma specifies the number of ALF APSsthat the slice refers to. The value of slice_num_alf_aps_ids_luma shallbe in the range of 0 to 6, inclusive. The_(:) maximum value maxVal ofthe truncated binary binarization tb(v) is set equal to 6.slice_alf_aps_id_chroma specifies file adaptation_parameter_set_id thatthe chroma component of the slice refers to. The TemporalId of the ALFAPS MAL unit having adaptation_parameter_set_id equal toslice_alf_aps_id_chroma shall be less than or equal to the TemporalId ofthe coded slice NAL unit.

Referring to the table above, slice_alf_aps_id_chroma may be signaledregardless of whether the slice type is not I or the number of ALF APSsreferenced by the slice is not one.

In addition, in an embodiment of the present disclosure, a method ofparsing/signaling alternative filter information for a chroma componentis proposed.

The table below exemplarily shows the syntax of the ALF data field basedon coding according to the present embodiment.

TABLE 31 alf_data( adaptation_parameter_set_id ) { Descriptor alf_luma_filter_signal_flag u(1)  alf_chroma_filter_signal_flag u(1) if( alf_luma _filter_signal_flag ) {   alf_luma_clip_flag u(1)  alf_luma_num_filters_signalled_minus1 tb(v)   if(alf_luma_num_filters_signalled_minus1 > 0 ) {    for( filtIdx = 0;filtIdx < NumAllFilters; filtIdx++ )     alf_luma_coeff_delta_idx[filtIdx ] tb(v)   }    }   }   alf_luma_coeff_signal_flag u(1)   if(alf_luma_coeff_signal_flag ) {    for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ )    alf_luma_coeff_flag[ sfIdx ] u(1)   }   for( sfIdx = 0; sfIdx <=alf_luma_num_filters_signalled_minus1; sfIdx++ ) {    if(alf_luma_coeff_flag[ sfIdx ] {     for( j = 0; j < 12; j++ )     alf_luma_coeff_abs[ sfIdx ][ j ] ue3(v)      if(alf_luma_coeff_abs[ sfIdx ][ j ] )       alf_luma_coeff_sign[ sfIdx ][ j] u(1)     }    }   }   if( alf_luma_clip_flag ) {    for( sfIdx = 0;sfIdx <= alf_luma_num_filters_signalled_minus1; sfIdx++ ) {     for ( j= 0; j < 12; j++ ) {        alf_luma_clip_idx[ sfIdx ][ j ] u(2)     }   }   }  }  if( alt_chroma_filter_fignal_flag ) {  alf_chroma_num_alts_minus1 tb(v)   for( altIdx = 0, altIdx <=alf_chroma_num_alts_minus1; altIdx++ ) {    alf_chroma_clip_flag[ altIdx] u(1)    for( j = 0; j < 6; j++ ) {     alf_chroma_coeff_abs[ altIdx ][j ] ue3(v)     if( alf_chroma_coeff_abs[ altIdx ][ j ] > 0 )     alf_chroma_coeff_sign[ altidx ][ j ] u(1)    }    if(alf_chroma_clip_flag[ altIdx ] ) {     for( j = 0; i < 6, j++ ) {     alf_chroma_clip _idx[ altIdx ][ j ] u(2)     }    }

The semantics of syntax elements included in the syntax of the tableabove may be expressed, for example, as shown in the following table.

TABLE 32   alf_luma_filter_signal_flag equal to 1 specifies that a lumafilter set is signalled. alf_luma_filter_signal_flag equal to 0specifies that a luma filter set is not signalled.alf_chroma_filter_signal_flag equal to 1 specifies that a chroma filteris signalled. alf_chroma_filter_signal_flag equal to 0 specifies that achroma filter is not signalled. The variable NumAlfFilters specifyingthe number of different adaptive loop filters is set equal to 25.alf_luma_clip_flag equal to 0 specifies that linear adaptive loopfiltering is applied on luma component. alf_luma_clip_flag equal to 1specifies that non-linear adaptive loop filtering may be applied on lumacomponent. alf_luma_num_filters_signalled_minus1 plus 1 specifies thenumber of adpative loop filter classes for which luma coefficients canbe signalled. The value of alf_luma_num_filters_signalled_ minus1 shallbe in the range of 0 to NumAlfFilters − 1, inclusive. The maximum valuemaxVal of the truncated binary binarization tb(v) is set equal toNumAlfFilters − 1. alf_luma_coeff_delta_idx[ filtIdx ] specifies theindices of the signalled adaptive loop filter luma coefficient deltasfor the filter class indicated by filtIdx ranging, from 0 toNumAlfFilters − 1. When alf_luma_coeff_delta_idx[ filtIdx ] is notpresent, it is inferred to be equal to 0. The maximum value maxVal ofthe truncated binary binarization th(v) is set equal toalf_luma_num_filters_signalled_minus1. alf_luma_coeff_signal_flag equalto 1 indicates that alf_luma_coeff_flag[ sfIdx ] is signalled.alf_luma_coeff_signal_flag equal to 0 indicates thatalf_luma_coeff_flag[ sfIdx ] is not signalled. alf_luma_coeff_flag[sfIdx ] equal 1 specifies that the coefficients of the luma filterindicated by sfIdx are signalled. alf_luma_coeff_flag[ sfIdx ] equal to0 specifies that all filter coefficients of the luma filter indicated bysfIdx are set equal to 0. When not present. alf_luma_coeff_flag[ sfIdx ]is set equal to 1. alf_luma_coeff_abs[ sfIdx ][ j ] specifies theabsolute value of the j-th coefficient delta of the signalled lumafilter indicated by sfIdx. When alf_luma_coeff_abs[ sfIdx ][ j ] is notpresent, it is inferred to be equal 0. alf_luma_coeff_sign[ sfIdx ][ j ]specifies the sign of the j-th luma coefficient of the filter indicatedby sfIdx as follows: − If alt_luma_coeff_sign[ sfIdx ][ j ] is equal to0, the corresponding luma filter coefficient  has a positive value. −Otherwise (alf_luma_coeff_sign[ sfIdx ][ j ] is equal to 1), thecorresponding luma filter  coefficient has a negative value. Whenalf_luma_coeff_sign[ sfIdx ][ j ] is not present, it is inferred to beequal to 0. The variable filtCoeff[ sfIdx ][ j ] with sfIdx =0..alf_luma_num_filters_signalled_minus1, j = 0..11 is initialized asfollows:  filtCoeff[ sfIdx ][ j ] = alf_luma_coeff_abs[ fIdx ][ j ] *  (1 − 2 * alf_luma_coeff_sign[ sfIdx ][ j ] )                (7−68) The luma  filter  coefficients  AlfCoeff_(L)[ adaptation_parameter_set_id ] with  elements AlfCoeff_(L)[ adaptation_parameter_set_id ][ filtIdx ][j ], with filtIdx = 0..NumAlfFilters − 1 and j = 0.11 are derived asfollows:  AlfCoeff_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ] =filtCoeff[ alf_luma_coeff_delta_  idx[ filtIdx ][ j ]           (7-70) AlfFixFiltCoeff =                          (7-72)   {    { 0, 0, 2, −3,1, −4, 1, 7, −1, 1, −1, 5}    { 0, 0, 0, 0, 0, −1, 0, 1, 0, 0, −1, 2}   { 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0}    { 0, 0, 0, 0, 0, 0, 0, 0, 0,0, −1, 1}    { 2, 2, −7, −3, 0, −5, 13, 22, 12, −3, −3, 17}    {−1, 0,6, −8, 1, −5, 1, 23, 0, 2, −5, 10}    { 0, 0, −1, −1, 0, −1, 2, 1, 0, 0,−1, 4}    { 0, 0, 3, −11, 1, 0, −1, 35, 5, 2, −9, 9}    { 0, 0, 8, −8,−2, −7, 4, 4, 2, 1, −1, 25}    { 0, 0, 1, −1, 0, −3, 1, 3, −1, 1, −1, 3}   { 0, 0, 3, −3, 0, −6, 5, −1, 2, 1, −4, 21}    {−7, 1, 5, 4, −3, 5,11, 13, 12, −8, 11, 12}    {−5, −3, 6, −2, −3, 8, 14, 15, 2, − 7, 11,16}    { 2, −1, −6, −5, −2, −2, 20, 14, −4, 0, −3, 25}    { 3, 1, −8,−4, 0, −8, 22, 5, −3, 2 −10, 29}    { 2, 1, −7, −1, 2, −11, 23, −5, 0,2, −10, 29}    {−6, −3, 8, 9, −4, 8, 9, 7 14, −2, 8, 9}    { 2, 1, −4,−7, 0, −8, 17, 22, 1, −1, −4, 23}    { 3, 0, −5, −7, 0, −7, 15, 18, −5,0, −5, 27}    { 2, 0, 0, −7, 1, −10, 13, 13, −4, 2, −7, 24}    { 3, 3,−13, 4, −2, −5, 9, 21, 25, −2, −3, 12}    {−5, −2, 7, −3, −7, 9, 8, 9,16, −2 , 15, 12}    { 0, −1, 0, −7, −5, 4, 11, 11, 8, −6, 12, 21}    {3, −2, −3, −8, −4, −1, 16, 15, −2, −3, 3, 26}    { 2, 1, −5, −4, −1, −8,16, 4, −2, 1, −7, 33}    { 2, 1, −4, −2, 1, −10, 17, −2, 0, 2, −11, 33}   { 1, −2, 7, −15, −16, 10, 8, 8, 20, 11, 14, 11}    { 2, 2, 3, −13, −13, 4, 8, 12, 2, −3, 16, 24}    { 1, 4, 0, −7, −8, −4, 9, 9, −2, −2, 8,29}    { 1, 1, 2, −4, −1, −6, 6, 3, −1, −1, −3, 10}    {−7, 3, 2, 10,−2, 3, 7, 11, 19, −7, 8, 10}    { 0, −2, −5, −3, 2, 4, 20, 15, −1, −3,−1, 22}    { 3, −1, −8, −4, −1, −4, 22, 8, −4, 2, −8, 28}    { 0, 3,−14, 3, 0, 1, 19, 17, 8, −3, −7, 20}    { 0, 2, −1, −8, 3, −6, 5, 21, 1,1, −9, 13}    {−4, −2, 8, 20, −2, 2, 3, 5, 21, 4, 6, 1}    { 2, −2, −3,−9, −4, 2, 14, 16, 3, 6, 8, 24}    { 2, 1, 5, −16, −7, 2, 3, 11, 15, −3,11, 22}    { 1, 2, 3, −11, −2, −5, 4, 8, 9, −3, −2, 26}    { 0, −1, 10,−9, −1, −8, 2, 3, 4, 0, 0, 29}    { 1, 2, 0, −5, 1, −9, 9, 3, 0, 1, −7,20}    {−2, 8, −6, −4, 3, −9, −8, 45, 14, 2, −13, 7}    { 1, −1, 16,−19, −8, −4, −3, 2, 19, 0, 4, 30}    { 1, 1, −3, 0, 2, −11, 15, −5, 1,2, −9, 24}    { 0, 1, −2, 0, 1, −4, 4, 0, 0, 1, −4, 7}    { 0, 1, 2, −5,1, −6, 4, 10, −2, 1, −4, 10}    { 3, 0, −3, −6, −2, −6, 14, 8, −1, −1,−3, 31}    { 0, 1, 0, −2, 1, −6, 5, 1, 0, 1, −5, 13}    { 3, 1, 9, −19,−21, 9, 7, 6, 13, 5, 15, 21}    { 2, 4, 3, −12, −13, 1, 7, 8, 3, 0, 12,26}    { 3, 1, −8, −2, 0, −6, 18, 2, −2, 3, −10, 23}    { 1, 1, 4, 1, 1,5, 8, 1, 1, 2, 5, 10}    { 0, 1, −1, 0, 0, −2, 2, 0, 0, 1, −2, 3}    {1, 1, −2, −7, 1, −7, 14, 18, 0, 0, −7, 21}    { 0, 1, 0, −2, 0, −7, 8,1, −2, 0, −3, 24}    { 0, 1, 1, −2, 2, −10, 10, 0, −2, 1, −7, 23}    {0, 2, 2, −11, 2, −4, −3, 39, 7, 1, 4 −10, 9}    { 1, 0, 13, −16, −5, −6,−1, 8, 6, 0, 6, 29}    { 1, 3, 1, −6, −4, −7, 9, 6, −3, −2, 3, 33}    {4, 0, −17, −1, −1, 5, 26, 8, −2, 3, − 15, 30}    { 0, 1, −2, 0, 2, −8,12, −6, 1, 1, −6, 16}    { 0, 0, 0, −1, 1, −4, 4, 0, 0, 0, −3, 11}    {0, 1, 2, −8, 2, −6, 5, 15, 0, 2, −7, 9}    { 1, −1, 12, −15, −7, −2, 3,6, 6, −1, 7, 30}   }, AlfClassToFiltMap =                         (7-73)   {    { 8, 2, 2, 2, 3, 4, 53, 9, 9, 52,4, 4, 5, 9, 2, 8, 10, 9, 1,   3, 39, 39, 10, 9, 52 }    { 11, 12, 13,14, 15, 30, 11, 17, 18, 19, 16, 20, 20, 4, 53, 21, 22, 23, 14, 25, 26,26,   27, 28, 10 }    { 16, 12, 31, 32, 14, 16, 30, 33, 53, 34, 35, 16,20, 4, 7, 16, 21, 36, 18, 19, 21, 26,   37, 38, 39 }    { 35, 11, 13,14, 43, 35, 16, 4, 34, 62, 35, 35, 30, 56, 7, 35, 21, 38, 24, 40, 16,21,   48, 57, 39 }    { 11, 31, 32, 43, 44, 16, 4, 17, 34, 45, 30, 20,20, 7, 5, 21, 22, 46, 40, 47, 26, 48,   63, 58, 10 }    { 12, 13, 50,51, 52, 11, 17, 53, 45, 9, 30, 4, 53, 19, 0, 22, 23, 25, 43, 44, 37, 27,  28, 10, 55 }    { 30, 33, 62, 51, 44, 20, 41, 56, 34, 45, 20, 41, 41,56, 5, 30, 56, 38, 40, 47, 11, 37,   42, 57, 8 }    { 35, 11, 23, 32,14, 35, 20, 4, 17, 18, 21, 20, 20, 20, 4, 16, 21, 36, 46, 25, 41, 26,  48, 49, 58 }    { 12, 31, 59, 59, 3, 33, 33, 59, 59, 52, 4, 33, 17,59, 55, 22, 36, 59, 59, 60, 22, 36,   59, 25, 55 }    { 31, 25, 15, 60,60, 22, 17, 19, 55, 55, 20, 20, 53, 19, 55, 22, 46, 25, 43, 60, 37, 28,  10, 55, 52 }    { 12, 31, 32, 50, 51, 11, 33, 53, 19, 45, 16, 4, 4,53, 5, 22, 36, 18, 25, 43, 26, 27,   27, 28, 10 }    { 5, 2, 44, 52, 3,4, 53, 45, 9, 3, 4, 56, 5, 0, 2, 5, 10, 47, 52, 3,   63, 39, 10, 9, 52 }   { 12, 34, 44, 44, 3, 56, 56, 62, 45, 9, 56, 56, 7, 5, 0, 22, 38, 40,47, 52, 48,   57, 39, 10, 9 }    { 35, 11, 23, 14, 51, 35, 20, 41, 56,62, 16, 20, 41, 56, 7, 16, 21, 38, 24, 40, 26, 26,   42, 57, 39 }    {33, 34, 51, 51, 52, 41, 41, 34, 62, 0, 41, 41, 56, 7, 5, 56, 38, 38, 40,44, 37, 42,   57, 39, 10 }    { 16, 31, 32, 15, 60, 30, 4, 17, 1 9, 25,22, 20, 4, 53, 19, 21, 22, 46, 25, 55, 26, 48,   63, 58, 55 }   }, It is  a  requirement  of  bitstream  conformance  that  the  values  ofAlfCoeff_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ] with filtIdx= 0..NumAlfFilters − 1, j = 0..11 shall be in the range of −2⁷ to 2⁷ −1,inclusive. It  is  a  requirement  of  bitstream  conformance  that  the values  of AlfCoeff_(L)[ adaptation_parameter_set_id ][ filtIdx ][ j ]with filtIdx = 0..NumAlfFilters − l, j = 0..11 shall be in the range of−2⁷ to 2⁷ − 1, inclusive. alf_luma_clip_idx[ sfIdx ][ j ] specifies theclipping index of the clipping value to use before multiplying by thej-th coefficient of the signalled luma filter indicated by sfIdx. Whenalf_luma_clip_idx[ sfIdx ][ j ] is not present, it is inferred to beequal 0 (no clipping). It is a requirement of bitstream conformance thatthe values of alf_luma_clip_ idx[ sfIdx ][ j ] with sfIdx =0..alf_luma_num_filters_signalled_minus1 and j = 0.11 shall be in therange of 0 to 3, inclusive. The variable filterClips[ sfIdx ][ j ] withsfIdx = 0..alf_luma_num_filters_signalled_minus1, j = 0..11 isinitialized as follows:  filterClips[ sfIdx ][ j ] = Round(2^(( BitDepthY * ( 4 − alf)_luma_clip_idx[sfIdx][ j ] ) / 4 ) )    (7-76) The luma filter clipping values AlfClip_(L)[adaptation_parameter_set_id ] with elements AlfClip_(L)[adaptation_parameter_set_id ][ filtIdx ][ j ], with filtIdx =0..NumAlfFilters − 1 and i = 0..11 are derived as follows:  AlfClip_(L)[adaptation_parameter_set_id ][ filtIdx ][ j ] = filterClips[alf_luma_coeff_delta_  idx[ filtIdx ] ][ j]                          (7-77) alf_chroma_num_alts_minus1 specifiesthe number of alternative filters for Chroma components. The maximumvalue maxVal of the truncated binary binarization tb(v) is set equal to8. alf_chroma_clip_flag[ altIdx ] equal to 0 specifies that linearadaptive loop filtering is applied on chroma components when using theChroma filter with index altIdx ranging  from 0 toalf_chroma_num_alts_minus1; alf_chroma_clip_flag[ altIdx ] equal to 1specifies that non-linear adaptive loop filtering is applied on chromacomponent when using  the Chroma filter with index altIdx. When notpresent, alf_chroma_clip_flag[ altIdx ] is inferred to be equal to 0.alf_chroma_coeff_abs[ altIdx ][ j ] specifies the absolute value of thej-th chroma filter coefficient of the alternative Chroma filter withindex altIdx, for altIdx ranging from 0 to alf_chroma_num_alts_minus1.When alf_chroma_coeff_abs[ altIdx ][ j ] is not present, it is inferredto be equal 0. It is a requirement of bitstream conformance that thevalues of alf_chroma_coeff_abs[ altIdx ][ j ] shall be in the range of 0to 2⁷ − 1, inclusive. alf_chroma_coeff_sign[ altIdx[ j ] specifies thesign of the j-th chroma filter coefficient of the alternative  Chroma filter  with  index  altIdx,  for  altIdx  ranging  from  0 toalf_chroma_num_alts_minus1, as follows: − If alf_chroma_coeff_sign[altIdx ][ j ] & equal to 0, the corresponding chroma filter  coefficienthas a positive value. − Otherwise (alf_chroma_coeff_sign[ altIdx ][ j ]is equal to 1), the corresponding chroma  filter coefficient has anegative value. When alf_chroma_coeff_sign[ altIdx ][ j ] is notpresent, it is inferred to be equal to 0. The chroma filter coefficientsAlfCoeff_(C)[ adaptation_parameter_set_id ][ altIdx ] with elementsAlfCoeff_(C)[ adaptation_parameter_set_id ][ altIdx ][ j ],              with altIdx − 0..alf_chroma_num_alts_minus1, j = 0..5 arederived as follows:  AlfCoeff_(C)[ adaptation_parameter_set_id ][ altIdx][ j ] = alf_chroma_coeff_abs[ altIdx ][ j ] *  ( 1 − 2 *alf_chroma_coeff_sign[ altIdx ][ j ] )                (7-82) It  is  a requirement  of  bitstream  conformance  that  the  values  ofAlfCoeff_(C)[ adaptation_parameter_set_id ][ altIdx ][ j ]             with altIdx = 0..alf_chroma_num_alts_minus1, j = 0..5 shallbe in the range of −2⁷ −1 to 2⁷ − 1, inclusive. alf_chroma_clip_idx[altIdx ][ j ] specifies the clipping index of the clipping value to usebefore multiplying by the i-th coefficient of the alternative Chromafilter with index  altIdx, for altIdx ranging from 0 toalf_chroma_num_alts_minus1. When alf_chroma_clip_ idx[ altIdx ][ j ] isnot present, it is inferred to be equal 0 (no clipping). It is arequirement of bitstream conformance that the values ofalf_chroma_clip_idx[ altIdx ][ j ] with j = 0..5 shall be in the rangeof 0 to 3, inclusive. The chroma filter clipping values AlfClip_(C)[adaptation_parameter_set_id ][ altIdx ] with el e m e n t s A l f C l ip _(C)[ adaptation_parameter_set_id[ altIdx ][ j ] , w i t h altIdx =0..alf_chroma_num_alts_minus1 and j = 0..5 are derived as follows: AlfClip_(C)[ adaptation_parameter_set_id ][ altIdx ][ j ] =  Round(2^(( BitDepthC − 8 ) *) 2^(( 8 * ( 3 − alf)_chroma_clip_idx[ altIdx][ j] ) / 3 ) )                                   (7−86)

Referring to the table above, the ALF data field may include alternativefilter information for the chroma component.

For example, the syntax element alf_chroma_num_alts_minus1 may indicatethe number of alternative filters for the chroma component. The indexaltIdx of the alternative filters for the chroma component may have avalue within the range of 0 to alf_chroma_num_alts_minus1. In addition,the syntax element alf_chroma_coeff_abs[altIdx][j] may indicate theabsolute value of the i-th coefficient included in the alternativefilter having the index altIdx. In addition, the syntax elementalf_chroma_coeff_sign[altIdx][j] may indicate the sign of the i-thcoefficient included in the alternative filter having the index altIdx.

In comparison with Table 32 above with Table 7 above, it can beunderstood that in an embodiment of the present disclosure, syntaxrelated to exponential Golomb coding of ALF coefficients and clippingcoefficients is simplified, and the syntax for predicting the filtercoefficients has been removed by using a fixed filter prediction. Inaddition, referring to Table 32 above, in an embodiment of the presentdisclosure, a method of signaling a fixed exponential Golomb orderinstead of signaling the order k of the exponential Golomb (EG) code isproposed.

The table below exemplifies the syntax of a coding tree unit (CTU).

TABLE 33 coding_tree_unit( ) { Descriptor  xCtb = ( CtbAddrInRs %PicWidthInCtbsY ) << CtbLog2SizeY  yCtb = ( CtbAddrInRs /PicWidthInCtbsY ) << CtbLog2SizeY  if( slice_sao_luma_flag | |slice_sao_chroma_flag )   sao( xCtb >> CtbLog2SizeY, yCtb > CtbLog2SizeY)  if( slice_alf_enabled_flag ){   alf_ctb_fag[ 0 ][ xCtb >>Log2CtbSize][ yCtb >> Log2CtbSize ] ae(v)   if( alf_ctb_flag[ 0 ][xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize] ) {     if(slice_num_alf_aps_ids_luma > 0)      alf_ctb_use_first_aps_flag ae(v)    if( !alf_ctb_use_first_aps_flag ) {      if(slice_num_alf_aps_ids_luma > 1 )       alf_use_aps_flag ae(v)      if(alf_use_aps_flag )       alf_luma_prev_filter_idx_minus1 ae(v)      else      alf_luma_fixed_filter_idx ae(v)     }    }   if(slice_alf_chroma_idc = = 1 | | slice_alf_chroma_idc = = 3 ) {   alf_ctb_flag[ 1 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2Ctbsize ] ae(v)   if( alf_ctb_flag[ 1 ][ xCtb >> LogCtbSize ][ yCtb >> LogCtbSize ]     && aps_alf_chroma_num_alts_minus1 > 0 )     alf_ctb_filter_alt_idx[1 ][ xCtb >> Log2CtbSize][ yCtb >> Log2CtbSize][ 0 ] ae(v)   }   if(slice_alf_chroma_idc = = 2 | | slice_alf_chroma_idc = = 3 ) {   alf_ctb_flag[ 2 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] ae(v)   if( alf_ctb_flag[ 2 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ]     && aps_alf_chroma_num_alts_minus1 > 0 )     alf_ctb_filter_alt_idx[2 ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ][ 0 ] ae(v)   }  }  if(slice_type = = I && qtbtt_dual_tree_intra_flag )  dual_tree_implicit_qt_split ( xCtb, yCtb, CtbSizeY, 0 )  else  coding_tree(xCtb, yCtb,CtbSizeY, CrbSizeY, 1, 0, 0, 0, 0, 0,SINGLE_TREE) }

The semantics of syntax elements included in the syntax of the tableabove may be expressed, for example, as shown in the following table.

TABLE 34 The array IsInSmr[ x ][ y ] specifying whether the sample at (x, y ) is located inside a shared merging candidate list region, isinitialized as follows for x = 0.. CtbsizeY − 1 and y = 0..CtbsizeY − 1: IsInSmr[ x ][ y ] = FALSE (7-96) alf_ctb_flag[ cIdx ][ xCtb >>Log2ctbSize ][ yCtb >> Log2CtbSize ] equal to 1 specifies that theadaptive loop filter is applied to the coding tree block of the colourcomponent indicated by cIdx of the coding tree unit at luma location (xCtb, yCtb ). alf_ctb_flag [ cIdx ][ xCtb >> Log2CtbSize ][ yCtb >>Log2CtbSize ] equal to 0 specifies that the adaptive loop filter is notapplied to the coding tree block of the colour component indicated bycIdx of the coding tree unit at luma location ( xCtb, yCtb ). Whenalf_ctb_flag[ cIdx ][ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] isnot present, it is inferred to be equal to 0. alf_cth_use_first_aps_flagequal to 1 sepeifies that the filter information in APS withadaptive_parameter_set_id equal to slice_alf_aps_id_luma[ 0 ] is used.alf_ctb_use_first_aps_flag equal to 0 specifies that the luma CTB doesnot use the filter information in APS with adaptive_parameter_set_idequal to slice_alf_aps_id_luma[ 0 ]. When alf_ctb_use_first_aps_flag isnot present, it is inferred to be equal to 0.   alf_use_aps_flag equalto 0 specifies that one of the fixed filter sets is applied to the lumaCTB.   alf_use_aps_flag equal to 1 specifies that a filter set fan anAPS is applied to the luma CTB.   When alf_use_aps_flag is not present,it is inferred to be equal to 0.   alf_luma_prev_filter_idx_minus1 plus1 specifies the previous filter that is applied to the luma   CTB. Thevalue of alf_luma_prev_filter_idx_minus1 shall be in a range of 0 to  slice_num_alf_aps_ids_luma − 2, inclusive.   The variableAlfCtbFiltSetIdxY[ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ]specifying   the filter set index for the luma CTB at location ( xCtb,yCtb ) is derived as follows:   -If  alf_ctb_use_first_aps  is  equal  to  1, AlfCtbFiltSetIdxY[ xCtb >>Log2CtbSize ][ yCtb >> Log2CtbSize ] is set equal to 16.   -Otherwise,  if  alf_use_aps_flag  is  equal  to  0, AlfCtbFiltSetIdxY[xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is set equal toalf_luma_fixed_filter_idx.   - Otherwise, AlfCtbFiltSetIdxY[ xCtb >>Log2CtbSize ][ yCtb >> Log2CtbSize ] is set equal to 17 +alf_luma_prev_filter_idx_minus1.   alf_luma_fixed_filter_idx specifiesthe fixed filter that is applied to the luma CTB. The value   ofalf_luma_fixed_filter_idx shall be in a range of 0 to 15, inclusive.  alf_ctb_filter_alt_idx[ 1 ][ xCtb >> Log2CtbSize ][ yCtb >>Log2CtbSize ][ sfIdx ] specify   the index of the alternative chromafilter to use when applying the adaptive loop filter on chroma  component with index 1 of the coding tree unit at luma location (xCtb, yCtb ). When   alf_ctb_filter_alt_idx[ 1 ][ xCtb >> Log2CtbSize ][yCtb >> Log2CtbSize ][ sfIdx ] is not   present, it is infered to beequal to zero.   alf_ctb_filter_alt_idx[ 2 ][ xCtb >> Log2CtbSize ][yCtb >> Log2CtbSize ][ sfIdx ] specify   the index of the alternativechroma filter to use when applying the adaptive loop filter on chroma  component with index 2 of the coding tree unit at luma location (xCtb, yCtb ). When   alf_ctb_filter_alt_idx[ 2 ][ xCtb >> Log2CtbSize ][yCtb >> Log2CtbSize ][ sfIdx ] is not   present, it is infered to beequal to zero.

Referring to the above table, index information of an alternative filterfor a coding tree unit (CTU) to which ALF is applied or a chromacomponent of a current block may be included. For example, the syntaxelement alf_ctb_filter_alt_idx may designate an index of an alternativefilter for a chroma component when ALF is applied to a chroma componenthaving a chroma index of 1 or 2.

The table below exemplifies syntax of sample adaptive offset syntax(SAO).

TABLE 35 sao( rx, ry ) { Descriptor  if( rx > 0 ) {   leftCtbInBrick =BrickId[ CtbAddrInBs ] = =       BrickId[ CtbAddrRsToBs[ CtbAddrInRs − 1] ]   if( lefCtbInBrick )    sao_merge_left_flag ae(v)  }  if( ry > 0 &&!sao_merge_left_flag ) {   upCtbInBrick = BrickId[ CtbAddrInBs ] = =      BrickId[ CtbAddrRsToBs[ CtbAddrInRs − PicWidthInCtbsY ] ]   if(upCtbInBrick )    sao_merge_up_flag ae(v)  }  if( !sao_merge_up_flag &&!sao_merge_left_flag )   for( cIdx = 0; cIdx < ( ChromaArrayType != 0 ?3 : 1 ); cIdx++ )    if( ( slice_sao_luma_flag && cIdx = = 0 ) | |     (slice_sao_chroma_flag && cIdx > 0 ) ) {     if( cIdx = = 0 )     sao_type_idx_luma ae(v)     else if( cIdx = = 1 )     sao_type_idx_chroma ae(v)     if( SaoTypeIdx[ cIdx ][ rx ][ ry ] !=0 ) {      for( i = 0; i < 4; i++ )      sao_offet_abs[ cIdx ][ rx ][ ry][ i ] ae(v)     if( SaoTypeIdx[ cIdx ][ rx ][ ry ] = = 1 ) {      for(i = 0; i < 4; i++ )       if( sao_offset_abs[ cIdx ][ rx ][ ry ][ i ] !=0 )        sao_offset_sign[ cIdx ][ rx ][ ry ][ i ] ae(v)     sao_band_position[ cIdx ][ rx ][ ry ] ae(v)     } else {      if(cIdx = = 0 ) ae(v)       sao_eo_class_luma      if( cIdx = = 1 )      sao_eo_class_chroma ae(v)     }    }   } }

The table below shows an example of the ALF procedure in a standarddocument format.

TABLE 36 Adaptive loop filter process General Inputs of this process arethe reconstructed picture sample array prior to adaptive loop filterrecPictureL and, when ChromaArrayType is not equal to 0, the arraysrecPictureCb and recPictureCr. Outputs of this process are the modifiedreconstructed picture sample array after adaptive loop filteralfPictureL and, when ChromaArrayType is not equal to 0, the arraysalfPictureCb and alfPictureCr. The sample values in the modifiedreconstructed picture sample array after adaptive loop filteralfPictureL and, when ChromaArrayType is not equal to 0, the arraysalfPictureCb and alfPictureCr are initially set equal to the samplevalues in the reconstructed picture sample array prior to adaptive loopfilter recPictureL and, when ChromaArrayType is not equal to 0, thearrays recPictureCb and recPictureCr, respectively. Whenslice_alf_enabled_flag is equal to 1, for every coding tree unit withluma coding tree block location ( rx, ry ), where rx = 0..PicWidthInCtbs− 1 and ry = 0..PicHeightInCtbs − 1, the following applies: Whenalf_ctb_flag[ 0 ][ rx ][ ry ] is equal to 1, the coding tree blockfiltering process for luma samples as specified in clause 8.8.5.2 isinvoked with recPictureL, alfPictureL, and the luma coding  tree  block  location  ( xCtb, yCtb )  set  equal  to  ( rx <<CtbLog2SizeY, ry << CtbLog2SizeY ) as inputs, and the output is themodified  filtered picture alfPictureL.  When ChromaArrayType is notequal to 0 and alf_ctb_flag[ 1 ][ rx ][ ry ] is equal to 1, the  codingtree block filtering process for chroma samples as specified in clause8.8.5.4 is invoked  with recPicture set equal to recPictureCb,alfPicture set equal to alfPictureCb, compIdx set equal  to 1, and thechroma coding tree block location ( xCtbC, yCtbC ) set equal to  ( ( rx<< CtbLogSizeY ) / SubWidthC, ( ry << CtbLog2SizeY ) / SubHeightC ) asinputs,  and the output is the modified filtered picture alfPictureCb. When ChromaArrayType is not equal to 0 and alf_ctb_flag[ 2 ][ rx ][ ry] is equal to 1, the  coding tree block filtering process for chromasamples as specified in clause 8.8.5.4 is invoked  with recPicture setequal to recPictureCr, alfPicture set equal to alfPictureCr, compIdx setequal  to 2, and the chroma coding tree block location ( xCtbC, yCtbC )set equal to  ( ( rx << CtbLog2SizeY ) / SubWidthC, ( ry << CtbLog2SizeY) / SubHeightC ) as inputs,  and the output is the modified filteredpicture alfPictureCr.

In addition, the table below shows an example of a coding tree unitfiltering procedure for luma samples in a standard document format.

TABLE 37 Coding tree block filtering process for luma samples Inputs ofthis process are: a reconstructed luma picture sample array recPictureLprior to the adaptive loop filtering process, a filtered reconstructedluma picture sample array alfPictureL, a luma location ( xCtb, yCtb )specifying the top-left sample of the current luma coding tree blockrelative to the top left sample of the current picture. Output of thisprocess is the modified filtered reconstructed luma picture sample arrayalfPictureL. The derivation process for filter index clause 8.8.5.3 isinvoked with the location ( xCtb, yCtb ) and the reconstructed lumapicture sample array recPictureL as inputs, and filtIdx[ x ][ y ] andtransposeIdx[ x ][ y ] with x, y = 0..CtbSizcY − 1 as outputs. For thederivation of the filtered reconstructed luma samples alfPictureL[ x ][y ], each reconstructed luma sample inside the current luma coding treeblock recPictureL[ x ][ y ] is filtered as follows with x, y =0..CtbSizeY − 1: The array of luma filter coefficients f[ j ] and thearray of luma clipping values c[ j ] corresponding to the filterspecified by filtIdx[ x ][ y ] is derived as follows with j = 0..11:  IfAlfCtbFiltSetIdxY[ xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize ] is lessthan 16, the  following applies:  i = AlfCtbFiltSetIdxY[ xCtb >>Log2CtbSize ][ yCtb >> Log2CtbSize ] (8-1172)  f[ j ] = AlfFixFiltCoeff[AlfClassToFiltMap[ i ][ filtidx ] ][ j ]  (8-1173)  c[ j]=2BitdepthY    (8-1174)  Otherwise (AlfCtbFiltSetIdx [ xCtb >>Log2CtbSize ][ yCtb >> Log2CtbSize ] is greater than  or equal to 16,the following applies:  i = slice_alf_aps_id_luma[ AlfCtbFiltSetIdxY[xCtb >> Log2CtbSize ][ yCtb >> Log2CtbSize  ] − 16](8-1175)  f[ j ] =AlfCoeffL[ i ][ filtIdx[ x ][ y ] ][ j ]    (8-1176)  c[ j ] = AlfClipL[i ][ filtIdx[ x ][ y ][ j ]   (8-1177)  The luma filter coefficients andclipping values index idx are derived depending on  transposeIdx[ x ][ y] as follows:  If transposeIndex[ x ][ y ] is equal to 1, the followingapplies:  idx[ ] = { 9, 4, 10, 8, 1, 5, 11 7, 3, 0, 2, 6 }    (8-1178) Otherwise, if transposeIndex[ x ][ y ] is equal to 2, the followingapplies:  idx[ ] = { 0, 3, 2, 1, 8, 7, 6, 5, 4, 9, 10, 11 }    (8-1179) Otherwise, if transposeIndex[ x ][ y ] is equal to 3, the followingapplies:  idx[ ] = { 9, 8, 10, 4, 3, 7, 11, 5, 1, 0, 2, 6 }    (8-1180) Otherwise, the following applies:  idx[ ] = { 0, 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11 }    (8-1181)  The locations ( hx + i, vy + j ) for each ofthe corresponding tana samples ( x, y ) inside the  given arrayrecPicture of luma samples with i, j = −3..3 are derived as follows: If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 and xCtb + x − PpsVirtualBoundariesPosX[ n ] is greater than or equal to 0and less than 3 for any  n = 0..pps_num_ver_virtual_boundaries − 1, thefollowing applies:  hx + i = Clip3( PpsVirtualBoundariesPosX[ n ],pic_width_in_luma_samples − 1,  xCtb + x + i )  (8-1182)  Otherwise, ifpps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1and  PpsVirtualBoundariesPosX[ n ]− xCtb − x is greater than 0 and lessthan 4 for any  n = 0..pps_num_ver_virtual_boundaries − 1, the followingapplies:  hx + i = Clip3( 0, PpsVirtualBoundariesPosX[ n ] − 1, xCtb +x + i )  (8-1183)  Otherwise, the following applies:  hx + i = Clip3( 0,pic_width_in luma_samples − 1, xCtb + x + i ) (8-1184) If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 and yCtb + y − PpsVirtualBoundariesPosY[ n ] is greater than or equal to 0and less than 3 for any  n − 0..pps_num_hor_virtual_boundaries − 1, thefollowing applies:  vy + j = Clip3( PpsVirtualBoundariesPosY[ n ],pic_height_in_in_luma_samples − 1,  yCtb + y + j )  (8-1185)  Otherwise,if pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1and  PpsVirtualBoundariesPosY[ n ] − yCtb − y is greater than 0 and lessthan 4 for any  n = 0..pps_num_hor_virtual_boundaries − 1, the followingapplies:  vy + j = Clip3( 0, PpsVirtualBoundariesPosY[ n ] − 1, yCtb +y + j )  (8-1186)  Otherwise, the following applies:  vy + j = Clip3( 0,pic_height_in_luma_samples − 1, yCtb + y + j ) (8-1187)  The variableapplyVirtualBoundary is derived as follows:  If one or more of thefollowing conditions are true, applyVirtualBoundary is set equal to 0: The bottom boundary of the current coding tree block is the bottomboundary of the picture.  The bottom boundary of the current coding treeblock is the bottom boundary of the brick and loop_filter_across_bricks_enabled_flag is equal to 0.  The bottomboundary of the current coding tree block is the bottom boundary of theslice and  loop_filter_across_slices_enabled_flag is equal to 0.  Thebottom boundary of the current coding tree block is one of the bottomvirtual boundaries of  the picture andpps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1. Otherwise, applyVirtualBoundary is set equal to 1.  The reconstructedsample offsets r1, r2 and r3 are specified in Table 8-22 according tothe  horizontal luma sample position y and applyVirtualBoundary.  Thevariable curr is derived as follows:  curr = recPictureL[ hx, vy]   (8-1188)  The variable sum is derived as follows:  sum = f[ idx[ 0 ]] * ( Clip3( −c[ idx[ 0 ] ], c[ idx[ 0 ] ], recPicture[ hx, vy + r3 ] −curr )  +    Clip3( −c[ idx[ 0 ] ], c[ idx[ 0 ] ], recPictureL[ hx, vy −r3 ] − curr ) ) +   f[ idx [ 1 ] ] * ( Clip3( −c[ idx[ 1 ] ], c[ idx[ 1] ], recPictureL[ hx + 1, vy + r2 ] −  curr ) +    Clip3( −c[ idx[ 1 ]], c[ idx[ 1 ] ], recPictureL[ hx − 1, vy − r2 ] − curr ) ) +   f[ idx[2 ] ] * ( Clips( −c[ idx[ 2 ] ], c[ idx[ 2 ] ], recPictureL[ hx, vy + r2] −  curr ) +    Clip3( −c[ idx[ 2 ] ], c[ idx[ 2 ] ], recPictureL[ hx,vy − r2 ] − curr ) ) +   f[ idx[ 3 ] ] * ( Clip3( −c[ idx[ 3 ] ], c[idx[ 3 ] ], recPictureL[ hx − 1, vy + r2 ] −  curr ) +    Clip3( −c[idx[ 3 ] ], c[ idx[ 3 ] ], recPictureL[ hx + 1, vy − r2 ] − curr ) ) +  f[ idx[ 4 ] ] * ( Clip3( −c[ idx[ 4 ] ], c[ idx[ 4 ] ],  recPictureL[hx + 2, vy + r1 ] −  curr ) +    Clip3( −c[ idx[ 4 ] ], c[ idx 4 ]], recPictureL[ hx − 2, vy − r1 ] − curr ) ) +   f[ idx[ 5 ] ] * (Clip3( −c[ idx[ 5 ] ], c[ idx[ 5 ] ], recPictureL[ hx, + 1, vy + r1 ] − curr ) +    Clip3( −c[idx[ 5 ] ], c[ idx[ 5 ] ], recPictureL[ hx − 1,vy − r1 ] − curr ) ) +   f[ idx[ 6 ] ] * ( Clip3( −c[ idx[ 6 ] ], c[idx[ 6 ] ], recPictureL[ hx, vy + r1 ] −  curr ) +    Clip3( −c[ idx[ 6] ], c[ idx[ 6 ] ], recPictureL[ hx, vy − r1 ] − curr ) ) +   f[ idx[ 7] ] * ( Clip3( −c[ idx[ 7 ] ], c[ idx[ 7 ] ], recPictureL[ hx − 1, vy +r1 ] −  curr ) +    Clip3( −c[ idx[ 7 ] ], c[ idx[ 7 ] ], recPictureL[hx + 1, vy − r1] − curr ) ) +   f[ idx[ 8 ] ] * ( Clip3( −c[ idx[ 8 ] ],c[ idx[ 8 ] ], recPictureL[ hx − 2, vy + r1 ] −  curr ) +    Clip3( −c[idx[ 8 ] ], c[ idx[ 8 ] ], recPictureL[ hx + 2, vy − r1 ] − curr ) ) +  f[ idx[ 9 ] ] * ( Clip3( −c[ idx[ 9 ] ], c[ idx[ 9 ] ], recPictureL[hx + 3, vy ] − curr )  +    Clip3( −c[ idx[ 9 ] ], c[ idx[ 9 ]], recPictureL[ hx − 3, vy ] − curr ) ) +   f[ idx[ 10 ] ] * ( Clip3(−c[ idx[ 10 ] ], c[ idx[ 10 ] ], recPictureL[ hx + 2, vy ] − curr )  +   Clip3( −c[ idx[ 10 ] ], c[ idx[ 10 ] ], recPictureL[ hx − 2, vy ] −curr ) ) +   f[ idx[ 11 ] ] * ( Clips3( −c[ idx[ 11 ] ], c[ idx[ 11 ] ],recPictureL[ hx + 1, vy ] − curr )  +    Clip3( −c[ idx[ 11 ] ], c[ idx[11 ] ], recPictureL[ hx − 1, vy ] − curr ) )  (8-1189)  sum = curr + ( (sum + 64 ) >> 7 )    (8-1190)  The modified filtered reconstructed lumapicture sample alfPictureL[ xCtb + x ][ yCtb + y ] is  derived asfollows:  If pcm_loop_filter_disabled_flag and pcm_flag[ xCtb + x ][yCtb + y ] are both equal to 1, the  following applies:  alfPictureL[xCtb + x ][ yCtb + y ] = recPictureL[ hx, vy ](8-1191)  Otherwise(pcm_loop_filter_disabled_flag is equal to 0 or pcm_flag[ x ][ y ] isequal 0), the  following applies:  alfPictureL[ xCtb + x ][ yCtb + y ] =Clip3( 0, ( 1 << BitDepthY ) − 1, sum )  (8-1192) Table8-22-Specification of r1, r2, and r3 according to the horizontal lumasample position y and applyVirtualBoundary condition r1 r2 r3 ( y == CtbSizeY − 5 | | y = = CtbSizeY − 4 ) && 0 0 0 ( applyVirtualBoundary= = 1 ) ( y = = CtbSizeY − 6 | | y = = CtbSizeY − 3 ) && 1 1 l (applyVirtualBoundary = = 1 ) ( y = = CtbSizeY − 7 | | y = = CtbSizeY − 2) && 1 2 2 ( applyVirtualBoundary = = 1 ) otherwise 1 2 3

In addition the table below shows an example of a procedure for derivingthe ALF transpose and filter index for luma samples in a standarddocument format.

TABLE 38 Derivation process for ALF transpose and filter index for lumasamples Inputs of this process are: a luma location ( xCtb, yCtb )specifying the top-left sample of the current luma coding tree blockrelative to the top left sample of the current picture, a reconstructedluma picture sample array recPictureL prior to the adaptive loopfiltering process. Outputs of this process are the classification filterindex array filtIdx[ x ][ y ] with x, y − 0..CtbSizeY − 1, the transposeindex array transposeIdx[ x ][ y ] with x, y = 0..CtbSizeY − 1. Thelocations ( hx + i, vy + j ) for each of the corresponding luma samples( x, y ) inside the given array recPicture of luma samples with i, j =−2..5 are derived as follows:If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 andxCtb + x − PpsVirtualBoundariesPosX[ n ] is greater than or equal to 0and less than 2 for any n = 0..pps_num_ver_virtual_boundaries − 1, thefollowing applies: hx + i = Clip3( PpsVirtualBoundariesPosX[ n ],pic_width_in_luma_samples − 1, xCtb + x + i )  (8-1193) Otherwise, ifpps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1and  PpsVirtualBoundariesPosX[ n ] − xCtb − x is greater than 0 and lessthan 6 for any  n = 0..pps_num_ver_virtual_boundaries − 1, the followingapplies:  hx + i = Clip3( 0, PpsVirtualBoundariesPosX[ n ] − 1, xCtb +x + i )  (8-1194)  Otherwise, the following applies:  hx + i = Clip3( 0,pic_width_in_luma_samples − 1, xCtb + x + i ) (8-1195) If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 and yCtb + y − PpsVirtualBoundariesPosY[ n ] is greater than or equal to 0and less than 2 for any  n = 0..pps_num_hor_virtual_boundaries − 1, thefollowing applies:  vy + j = Clip3( PpsVirtualBoundariesPosY[ n ],pic_height_in_luma_samples − 1, yCtb + y +  j )  (8-1196)  Otherwise, ifpps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1and  PpsVirtualBoundariesPosY[ n ] − yCtb − y is greater than 0 and lessthan 6 for any  n = 0..pps_num_hor_virtual_boundaries − 1, the followingapplies:  vy + j − Clip3( 0, PpsVirtualBoundariesPosY[ n ] − 1, yCtb +y + j )  (8-1197)  Otherwise, the following applies:  If yCtb + CtbSizeYis greater than or equal to pic_height_in_luma_samples, the following applies:  vy + j = Clip3( 0, pic_height_in_luma_samples − 1, yCtb + y +j ) (8-1198)  Otherwise, if y is less than CtbSizeY − 4, the followingapplies:  vy + j = Clip3( 0, yCtb + CtbSizeY − 5, yCtb + y + j) (8-1199)  Otherwise, the following applies:  vy + j = Clip3( yCtb +CtbSizeY − 4, pic_height_in_luma_samples − 1, yCtb + y + j )    (8-1200) The classification filter index array filtIdx and the transpose indexarray transposeIdx are derived  by the following ordered steps: The variables filtH[ x ][ y ], filtV[ x ][ y ], filtD0[ x ][ y] and filtD1[ x ][ y ] with  x, y = −2..CtbSizeY + 1 are derived asfollows:  If both x and y are even numbers or both x and y are unevennumbers, the following applies:  filtH[ x ][ y ] = Abs( ( recPicture[hx, vy ] << 1) − recPicture[ hx − 1, vy ]− (8-1201)     recPicture[ hx +1, vy ] )  filtV[ x ][ y ] = Abs( ( recPicture hx, vy ] << 1 ) −recPicture[ hx, vy − 1 ]− (8-1202)     recPicture[ hx, vy + 1 ] ) filtD0[ x ][ y ] = Abs( ( recPicture[ hx, vy ] << 1 ) − recPicture[ hx− 1,vy − 1 ] −   recPicture[ hx + 1, vy + 1 ] ) (8-1203)  filtD1[ x ][ y] = Abs( ( recPicture[ hx, vy ] << 1 ) − recPicture [ hx + 1, vy − 1 ] −  recPicture[ hx − 1, vy + 1 ] ) (8-1204)  Otherwise, filtH[ x ][ y ],filtV[ x ][ y ], filtD0[ x ][ y ] and filtD1[ x ][ y ] are set equal to0.  The variables minY, maxY and ac are derived as follows:  If ( y << 2) is equal to ( CtbSizeY − 8 ) and ( yCtb + CtbSizeY ) is less than pic_height_in_luma_samples − 1, minY is set equal to −2, maxY is setequal to 3 and ac is set  equal to 96.  Otherwise, if ( y << 2 ) isequal to ( CtbSizeY − 4) and ( yCtb + CtbSizeY ) is less than pic_height_in_luma_samples − 1, minY is set equal to 0, maxY is setequal to 5 and ac is set  equal to 96.  Otherwise, minY is set equal to−2 and maxY is set equal to 5 and ac is set equal to 64. The variables varTempH1[ x ][ y ], varTempV1[ x ][ y ], varTempD01[ x][ y ],  varTempD11[ x ][ y ] and varTemp[ x ][ y ] with x, y = 0..(CtbSizeY − 1 ) >> 2 are derived as  follows:  sumH[ x ][ y ] =ΣiΣj  filtH[ h(x << 2 ) + i − xCtb ][ v(y << 2) + j − yCtb ]  with  i =−2..5, j = minY..maxY  (8-1205)  sumV[ x ][ y ] = ΣiΣj  filtV[ h(x << 2) + i − xCtb ][ v(y << 2) + j − yCtb ]  with  i = −2..5, j =minY..maxY  (8-1206)  sumD0[ x ][ y ] = ΣiΣj  filtD0[ h(x << 2 ) + i −xCtb ][ v(y << 2) + j − yCtb ]  with  i = −2..5, j =minY..maxY  (8-1207)  sumD1[ x ][ y ] = ΣiΣj  filtD1[ h(x << 2 ) + i −xCtb ][ v(y << 2) + j − yCtb ]  with  i = −2..5, j =minY..maxY  (8-1208)  sumOfHV[ x ][ y ] = sumH[ x ][ y ] + sumV[ x ][ y]  (8-1209)  The valables dir1[ x ][ y ], dir2[ x ][ y ] and dirS[ x ][y ] with x, y = 0..CtbSizeY − 1 are  derived as follows:  The variableshv1, hv0 and dirHV are derived as follows:  If sumV[ x >> 2 ][ y >> 2 ]is greater than sumH[ X >> 2 ][ y >> 2 ], the following applies:  hv1 =sumV[ x >> 2 ][ y >> 2 ]   (8-1210)  hv0 = sumH[ x >> 2 ][ y >> 2]   8-1210)  dirHV = 1    (8-1212)  Otherwise, the following applies: hv1 = svmH[ x >> 2 ][ y >> 2 ]   (8-1213)  hv0 = sumV[ x >> 2 ][ y >> 2]   (8-1214)  dirHV = 3    (8-1215)  The variables d1, d0 and dirD arederived as follows:  If sumD0[ x >> 2 ][ y >> 2 ] is greater than sumD1[x >> 2 ][ y >> 2 ], the following applies:  d1 = sumD0[ x >> 2][ y >> 2]   (8-1216)  d0 = sumD1[ x >> 2 ][ y >> 2 ]   (8-1217)  dirD =0    (8-1218)  Otherwise, the following applies:  d1 = sumD1[ x >> 2 ][y >> 2 ]   (8-1219)  d0 = sumD0[ x >> 2 ][ y >> 2 ]   (8-1220)  dirD =2    (8-1221)  The variables hvd1, hvd0, are derived as follows:  hvd1 =( d1 * hv0 > hv1 * d0 ) ? d1 : hv1   (8-1222)  hvd0 = ( d1 * hv0 > hv1 *d0 ) ? d0 : hv0   (8-1223)  The variables dirS[ x ][ y ], dir1[ x ][ y ]and dir2[ x ][ y ] derived as follows:  dir1[ x ][ y ] = ( d1 * hv0 >hv1 * d0 ) ? dirD : dirHV (8-1224)  dir2[ x ][ y ] = ( d1 * hv0 > hv1 *d0 ) ? dirHV : dirD (8-1225)  dirS[ x ][ y ] = ( hvd1 > 2 * hvd0 ) ? 1 :( ( hvd1 * 2 > 9 * hvd0 ) ? 2 : 0 )  (8-1226)  The variable avgVar[ x ][y ] with x, y = 0..CtbSizeY − 1 is derived as follows:  varTab[ ] = { 0,1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4 }  (8-1227)  avgVar[ x ][ y] = varTab[ Clip3( 0, 15, ( sumOfHV[ x >> 2 ][ y >> 2 ] * ac ) >> ( 3 +BitDept  hY ) ) ] (8-1228)  The classification filter index arrayfiltIdx[ x ][ y ] and the transpose index array  transposeIdx[ x ][ y ]with x = y = 0..CtbSizeY − 1 are derived as follows:  transposeTable[ ]= { 0, 1, 0, 2, 2, 3, 1, 3 }  transposeIdx[ x ][ y ] = transposeTable[dir1[ x ][ y ] * 2 + ( dir2[ x ][ y ] >> 1 ) ]  filtIdx[ x ][ y ] =avgVar[ x ][ y ]  When dirS[ x ][ y ] is not equal 0, filtIdx[ x ][ y ]is modified as follows:  filtIdx[ x ][ y ] += ( ( ( dir1[ x ][ y ] & 0x1) << 1 ) + dirS[ x ][ y ] ) * 5 (8-1229)

In addition, the table below shows an example of a coding tree unitfiltering procedure for chroma samples in a standard document format.

TABLE 39 Coding tree block filtering process for chroma samples Inputsof this process are: a reconstructed chroma picture sample arrayrecPicture prior to the adaptive loop filtering process, a filteredreconstructed chroma picture sample array alfPicture, a component indexcompIdx specifying the chroma component index of the current chromacoding treeblock, a chroma location ( xCtbC, yCtbC ) specifying thetop-left sample of the current chroma coding tree block relative to thetop left sample of the current picture. Output of this process is themodified filtered reconstructed chroma picture sample array alfPicture.The width and height of the current chroma coding tree block ctbWidthCand ctbHeightC is derived as follows: ctbWidthC = CtbSizeY /SubWidthC   (8-1230) ctbHeightC = CtbSizeY / SubHeightC   (8-1231)  Forthe derivation of the filtered reconstructed chroma samples alfPicture[x ][ y ], each  reconstructed chroma sample inside the current chromacoding tree block recPicture[ x ][ y ] is  filtered as follows with x =0..ctbWidthC − 1, y = 0..ctbHeightC − 1:  The locations ( hx + i, vy + j) for each of the corresponding chroma samples ( x, y ) inside the given array recPicture of chroma samples with i, j = −2..2 are derivedas follows: If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 and xCtbC + X − PpsVirtualBoundariesPosX[ n ] / SubWidthC is greater thanor equal to 0 and less  than 2 for any n =0..pps_num_ver_virtual_boundaries − 1, the following applies:  hx + i =Clip3( PpsVirtualBoundariesPosX[ n ] / SubWidthC, (8-1232)   pic_width_in_luma_samples / SubWidthC − 1, xCtbC + x + i ) Otherwise, if pps_loop_filter_across_virtual_boundaries_disabled_flagis equal to 1 and  PpsVirtualBoundariesPosX[ n ] / SubWidthC − xCtbC − xis greater than 0 and less than 3 for  any n =0..pps_num_ver_virtual_boundaries − 1, the following applies:  hx + i =Clip3( 0, PpsVirtualBoundariesPosX[ n ] / SubWidthC − 1, xCtbC + x + i )  (8-1233)  Otherwise, the following applies:  hx + i = Clip3( 0,pic_width_in_luma_samples / SubWidthC − 1, xCtbC + x + i ) (8-1234) If pps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1 and yCtbC + y − PpsVirtualBoundariesPosY[ n ] / SubHeightC is greater thanor equal to 0 and less  than 2 for any n =0..pps_num_hor_virtual_boundaries − 1, the following applies:  vy + j =Clips3( PpsVirtualBoundariesPosY[ n ] / SubHeightC , (8-1235)   pic_height_in_luma_samples / SubHeightC − 1, yCtbC + y + j ) Otherwise, if pps_loop_filter_across_virtual_boundaries_disabled_flagis equal to 1 and  PpsVirtualBoundariesPosY[ n ] / SubHeightC − yCtbC −y is greater than 0 and less than 3 for  any n =0..pps_num_hor_virtual_boundaries − 1, the following applies:  vy + j =Clip3( 0, PpsVirtualBoundariesPosY[ n ] / SubHeightC − 1, yCtbC + y + j)   (8-1236)  Otherwise, the following applies:  vy + j = Clip3( 0,pic_height_in_luma_samples / SubHeightC − 1, yCtbC + y + j ) (8-1237) The variable applyVirtualBoundary is derived as follows:  If one ormore of the following conditions are true, applyVirtualBoundary is setequal to 0:  The bottom boundary of the current coding tree block is thebottom boundary of the picture.  The bottom boundary of the currentcoding tree block is the bottom boundary of the brick and loop_filter_across_bricks_enabled_flag is equal to 0.  The bottomboundary of the current coding tree block is the bottom boundary of theslice and  loop_filter_across_slices_enabled_flag is equal to 0.  Thebottom boundary of the current coding tree block is one of the bottomvirtual boundaries of  the picture andpps_loop_filter_across_virtual_boundaries_disabled_flag is equal to 1. Otherwise, applyVirtualBoundary is set equal to 1.  The reconstructedsample offsets r1 and r2 are specified in Table 8-22 according to thehorizontal  luma sample position y and applyVirtualBoundary.  Thevariable curr is derived as follows:  curr = recPicture[ hx, vy]    (8-1238)  The variable altIdx is derived as follows:  altIdx = alf_ctb_filter_alt_idx[ compIdx ][ xCtb >> Log2CtbSize ][ yCtb >>Log2CtbSize ][ 0 ]   (8-1239)  The array of chroma filter coefficientsf[ j ] and the array of chroma clipping values c[ j ] is  derived asfollows with j = 0..5:  f[ j ] = AlfCoeffC[ slice_alf_aps_id_chroma ][altIdx ][ j ] (8-1239)  c[ j ] =AlfClipC[ slice_alf_aps_id_chroma ][altIdx ][ j ] (8-1240)  The variable sum is derived as follows:  sum =f[ 0 ] * ( Clip3( −c[ 0 ], c[ 0 ], recPicture[ hx, vy + r2 ] − curr ) +   Clip3( −c[ 0 ], c[ 0 ], recPicture[ hx, vy − r2 ] − curr ) ) +   f[ 1] * ( Clip3( −c[ 1 ], c[ 1 ], recPicture[ hx + 1, vy + r1 ] − curr ) +   Clip3( −c[ 1 ], c[ 1 ], recPicture[ hx − 1, vy − r1 ] − curr ) ) +  f[ 2 ] * ( Clip3( −c[ 2 ], c[ 2 ], recPicture[ hx, vy + r1 ] − curr) +    Clip3( −c[ 2 ], c[ 2 ], recPicture[ hx, vy − r1 ] − curr ) )+  (8-1241)   f[ 3 ] * ( Clip3( −c[ 3 ], c[ 3 ], recPicture[ hx − 1,vy + r1 ] − curr ) +    Clip3( −c[ 3 ], c[ 3 ], recPicture[ hx + 1, vy −r1 ] − curr ) ) +   f[ 4 ] * ( Clip3( −c[ 4 ], c[ 4 ], recPicture[ hx +2, vy ] − curr ) +    Clip3( −c[ 4 ], c[ 4 ], recPicture[ hx − 2, vy ] −curr ) ) +   f[ 5 ] * ( Clip3( −c[ 5 ], c[ 5 ], recPicture[ hx + 1, vy ]− curr ) +    Clip3( −c[ 5 ], c[ 5 ], recPicture[ hx − 1, vy ] − curr ))  sum = curr + ( sum + 64 ) >> 7 )  (8-1242)  The modified filteredreconstructed chroma picture sample alfPicture[ xCtbC + x ][ yCtbC + y ] is derived as follows:  If pcm_loop_filter_disabled_flag and  pcm_flag[( xCtbC + x ) * SubWidthC ][ ( yCtbC + y ) * SubHeightC ] are both equalto 1, the  following applies:  alfPicture[ xCtbC + x ][ yCtbC + y ] =recPictureL[ hx, vy ]  (8-1243)  Otherwise(pcm_loop_filter_disabled_flag is equal to 0 or pcm_flag[ x ][ y ] isequal 0), the  following applies:  alfPicture[ xCtbC + x ][ yCtbC + y ]= Clip3( 0, ( 1 << BitDepthC ) − 1, sum )  (8-1244) Table8-23-Specification of r1 and r2 according to the horizontal luma sampleposition y and applyVirtualBoundary condition r1 r2 ( y = = ctbHeightC −2 | | y = = ctbHeightC − 3 ) && 0 0 ( applyVirtualBoundary = = 1 ) ( y == ctbHeightC − 1 | | y = = ctbHeightC − 4 ) && 1 1 (applyVirtualBoundary = = 1 ) otherwise: 1 2

In addition, the table below shows an example of an initialization value(initValue) and a shift index (shiftIdx) according to a context indexincrement (ctxInc) for the syntax element alf_ctb_filter_alt_idx.

TABLE 40 initValue of alf_ctb_filter_alt_idx ctxInc initType = = 0initType = = 1 initType = = 2 shiftIdx 0 — — — — 1 100 153 200 0 2 100153 200 0

Referring to the table above, the initialization value (initValue) forthe syntax element alf_ctb_filter_alt_idx indicating index informationof the alternative filter may be determined based on the context indexincrement (ctxInc) or the initialization type (initType) foralf_ctb_filter_alt_idx. In addition, the shift index (shiftIdx) foralf_ctb_filter_alt_idx may be determined based on the context indexincrement (ctxInc) for alf_ctb_filter_alt_idx.

In addition, the table below shows an example of binarization related tovarious syntaxes including the syntax element alf_ctb_filter_alt_idx.

TABLE 41 Binarization Syntax structure Syntax element Process Inputparameters slice_data( ) end_of_brick_one_bit FL cMax = 1coding_tree_unit( ) alf_ctb_flag[ ][ ][ ] FL cMax = 1alf_ctb_use_first_aps_flag FL cMax = 1 alf_use_aps_flag FL cMax = 1alf_luma_fixed_filter_idx TB cMax = 15 alf_luma_prev_filter_idx_minus1TB cMax = slice_num_alf_aps_ids_luma − 2 alf_ctb_filter_alt_idx[ 1 ][ ][][ ] TR cMax = alf_chroma_num_alts_minus1, cRiceParam = 0alf_ctb_filter_alt_idx[ 2 ][ ][ ][ ] TR cMax =alf_chroma_num_alts_minus1, cRiceParam = 0 sao( ) sao_merge_left_flag FLcMax = 1 sao_merge_up_flag FL cMax = 1 sao_type_idx_luma TR cMax = 2,cRiceParam = 0 sao_type_idx_chroma TR cMax = 2, cRiceParam = 0sao_offset_abs[ ][ ][ ][ ] TR cMax = ( 1 << (Min(bitDepth, 10) − 5 ) ) −1, cRiceParam = 0 sao_offset_sign[ ][ ][ ][ ] FL cMax = 1 sao_band_position[ ][ ][ ] FL cMax = 31 sao_eo_class_luma FL cMax = 3sao_eo_class_chroma FL cMax = 3 coding_tree( ) split_cu_flag FL cMax = 1split_qt_flag FL cMax = 1 mtt_split_cu_vertical_flag FL cMax = 1mtt_split_cu_binary_flag FL cMax = 1 coding_unit( ) cu_skip_flag[ ][ ]FL cMax = 1 pred_mode_ibc_flag FL cMax = 1 pred_mode_flag FL cMax = 1pcm_flag[ ][ ] FL cMax = 1 intra_bdpcm_flag[ ][ ] FL cMax = 1intra_bdpcm_dir_flag[ ][ ] FL cMax = 1 intra_mip_flag[ ][ ] FL cMax = 1intra_mip_mpm_flag[ ][ ] FL cMax = 1 intra_mip_mpm_idx[ ][ ] TR cMax =2, cRiceParam = 0 intra_mip_mpm_remainder[ ][ ] FL cMax = (cbWidth = = 5&& cbHeight = = 4) ? 31 : ( (cbWidth <= 8 && cbHeight <= 8) ? 15 : 7)intra_luma_ref_idx[ ][ ] TR cMax = 2, cRiceParam = 0intra_subpartitions_mode_flag FL cMax = 1 itnra_subpartitions_split_flagFL cMax = 1 intra_luma_mpm_flag[ ][ ] FL cMax = 1intra_luma_not_planar_flag[ ][ ] FL cMax = 1 intra_luma_mpm_idx[ ][ ] TRcMax = 4, cRiceParam = 0 intra_luma_mpm_remainder[ ][ ] TB —intra_chroma_pred_mode[ ] 9.5.3.8 cMax = 1 general_merge_flag[ ][ ] FLcMax = 1 inter_pred_idc[ x0 ][ y0 ] 9.5.3.9 cbWidth, cbHeightinter_affine_flag[ ][ ] FL cMax = 1 cu_affine_type_flag[ ][ ] FL cMax =1 sym_mvd_flag[ ][ ] FL cMax = 1 ref_idx_l0[ ][ ] TR cMax =NumRefIdxActive[ 0 ] − 1, cRiceParam = 0 mvp_l0_flag[ ][ ] FL cMax = 1ref_idx_l1[ ][ ] TR cMax = NumRefIdxActive[ 1 ] − 1,cRiceParam = 0mvp_l1_flag[ ][ ] FL cMax = 1 avmr_flag[ ][ ] FL cMax = 1amvr_precision_flag[ ][ ] FL cMax = 1 bcw_idx[ ][ ] TR cMax =NoBackwardPredFlag ? 4: 2 cu_cbf FL cMax = 1 cu_sbt_flag FL cMax = 1cu_sbt_quad_flag FL cMax = 1 cu_sbt_horizontal_flag FL cMax = 1cu_sbt_pos_flag FL cMax = 1 merge_data( ) regular_merge_flag[ ][ ] FLcMax = 1 mmvd_merge_flag[ ][ ] FL cMax = 1 mmvd_cand_flag[ ][ ] FL cMax= 1 mmvd_distance_idx[ ][ ] TR cMax = 7, cRiceParam = 0mmvd_direction_idx[ ][ ] FL cMax = 3 ciip_flag[ ][ ] FL cMax = 1merge_subblock_flag[ ][ ] AL cMax = 1 merge_subblock_idx[ ][ ] TR cMax =MaxNumSubblockMergeCand − 1, cRiceParam = 0 merge_triangle_split_dir[ ][] FL cMax = 1 merge_triangle_idx0[ ][ ] TR cMax =MaxNumTriangleMergeCand − 1, cRiceParam = 0 merge_triangle_idx1[ ][ ] TRcMax = MaxNumTriangleMergeCand − 2, cRiceParam = 0 merge_idx[ ][ ] TRcMax = MaxNumMergeCand − 1, cRiceParam = 0 mvd_coding( )abs_mvd_greater0_flag[ ] FL cMax = 1 abs_mvd_greater1_flag[ ] FL cMax =1 abs_mvd_minus2[ ] EG1 — mvd_sign_flag[ ] FL cMax = 1 transtorm_unit( )tu_cbf_luma[ ][ ][ ] FL cMax = 1 tu_cbf_cb[ ][ ][ ] FL cMax = 1tu_cbf_cr[ ][ ][ ] FL cMax= 1 cu_qp_delta_abs 9.5.3.10 —cu_qp_delta_sign_flag FL cMax = 1 transform_skip_flag[ i ] FL cMax = 1tu_mts_idx[ ][ ] TR cMax = 4, cRiceParam = 0 tu_joint_cbcr_residual[ ][] FL cMax = 1 residual_coding( ) last_sig_coeff_x_prefix TR cMax = (log2ZoTbWidth << 1 ) − 1, cRiceParam = 0 last_sig_coeff_y_prefix TR cMax= ( log2ZoTbHeight << 1 ) − 1, cRiceParam = 0 last_sig_coeff_x_suffix FLcMax = ( 1 << ( ( last_sig_coeff_x_prefix >> 1 ) −1 ) − 1 )last_sig_coeff_y_suffix FL cMax = ( 1 << ( ( last_sig_coeff_y_prefix >>1 ) − 1 ) − 1 ) coded_sub_block_flag[ ][ ] FL cMax = 1 sig_coeff_flag[][ ] FL cMax = 1 par_level_flag[ ] FL cMax = 1 abs_level_gtx_flag[ ][ ]FL cMax = 1 abs_remainder[ ] 9.5.3.11 cIdx, current sub-block index i,x0, y0 dec_abs_level[ ] 9.5.3.12 cIdx, x0, y0, xC, yC, log2TbWidth,log2TbHeight coeff_sign_flag[ ] Fl cMax = 1

Referring to the table above, the syntax element alf_ctb_filter_alt_idxindicating index information of the alternative filter for the codingtree unit (CTU) to which the ALF is applied or the chroma component ofthe current block may be coded based on a truncated rice binarizationmethod of the index parameter of the alternative filter. In addition, amaximum value (cMax) of the index parameter of the alternative filterfor the chroma component may correspond to alf_chroma_num_alts_minus1.alf_chroma_num_alts_minus1 may correspond to a value that is one lessthan the number of alternative filters for the chroma component. Inaddition, the truncated rice binarization method may be performed basedon a rice parameter (cRiceParam) value of 0.

In addition, the table below shows an example of a procedure forderiving a context index increment (ctxInc) for the syntax elementalf_ctb_filter_alt_idx in a standard document format.

TABLE 42   1.1.1.2.1 Derivation process of ctxInc for the syntax elementalf_ctb_filter_alt_idx Input of this process is the colour componentindex cIdx. Output of this process is the variable ctxInc. The variablectxInc is derived as follows: ctxInc = cIdx        (9-50)

Bins of a bin string for the index information of the alternative filtermay be entropy-decoded based on the context model, and the context modelfor the bins of the bin string for the index information of thealternative filter may be determined based on the context indexincrement for the bins. Referring to the table above, the context modelfor the index information of the alternative filter may be determinedbased on the context index increment for the syntax elementalf_ctb_filter_alt_idx indicating the index information of thealternative filter, and the context index increment for the indexinformation of the alternative filter may be determined based on the CTUto which the ALF is applied or the chroma component of the currentblock. For example, the context index increment for the indexinformation of the alternative filter may be determined based on theindex of the chroma component. For example, the context index incrementderived when the chroma component of the current block is cb may bedifferent from the context index increment derived when the chromacomponent of the current block is cr. For example, when the index of thechroma component is 0, the context index increment foralf_ctb_filter_alt_idx may have a value of 0, and when the index of thechroma component is 1, the context index increment foralf_ctb_filter_alt_idx may have a value of 1. Alternatively, forexample, when the index of the chroma component is 1, the context indexincrement for alf_ctb_filter_alt_idx may have a value of 1, and when theindex of the chroma component is 2, the context index increment foralf_ctb_filter_alt_idx may have a value of 2.

FIGS. 13 and 14 schematically show an example of a video/image encodingmethod and related components according to embodiment(s) of the presentdisclosure. The method illustrated in FIG. 13 may be performed by theencoding apparatus illustrated in FIG. 2 . Specifically, for example,S1300 may be performed by the predictor 220 of the encoding apparatus,S1310 may be performed by the residual processor 230 of the encodingapparatus, and S1320 may be performed by the residual processor 230 orthe adder 250 of the encoding apparatus. In addition, S1330 to S1340 maybe performed by the adder 250 of the encoding apparatus, and S1350 maybe performed by the entropy encoder 240 of the encoding apparatus. Themethod illustrated in FIG. 13 may include the embodiments describedabove in the present disclosure.

Referring to FIG. 13 , the encoding apparatus derives prediction samplesfor a current block in a current picture (S1300). The encoding apparatusmay derive prediction samples of the current block based on a predictionmode. The encoding apparatus may generate prediction mode informationindicating a prediction mode applied to the current block. In this case,various prediction methods described in the present disclosure, such asinter prediction or intra prediction, may be applied. In this case, theencoding apparatus may generate prediction mode information. Theencoding apparatus may generate prediction mode information indicating aprediction mode applied to the current block.

The encoding apparatus derives residual samples for a current blockbased on prediction samples (S1210). The encoding apparatus may deriveresidual samples based on the prediction samples and the originalsamples. For example, the encoding apparatus may derive residual samplesbased on a comparison between the original samples and the modifiedreconstructed samples.

The encoding apparatus generates residual information and reconstructedsamples based on residual samples (S1320). Based on the residualinformation, (modified) residual samples may be derived. Reconstructedsamples may be generated based on the (modified) residual samples andthe prediction samples.

The encoding apparatus derives filter coefficients for an adaptive loopfilter (ALF) for reconstructed samples (S1330). The encoding apparatusmay generate ALF-related information. The encoding apparatus may derivean ALF-related parameter that may be applied for filtering thereconstructed samples.

In addition, the encoding apparatus generates ALF information based onfilter coefficients (S1340). For example, the ALF information mayinclude information on the ALF filter coefficients described above inthe present disclosure, information on a fixed filter, information onclipping, and the like. Alternatively, for example, the ALF informationmay include alternative filter information for the chroma component ofthe current block.

The encoding apparatus encodes the image information including ALFinformation and residual information (S1250). The image/videoinformation may include information for generating the reconstructedsamples and/or information related to the ALF. The information forgenerating the reconstructed samples may include, for example,prediction-related information, residual information, and/orquantization/transformation-related information. The prediction-relatedinformation may include information on various prediction modes (e.g.,merge mode, MVP mode, etc.), MVD information, and the like.

The encoded image/video information may be output in the form of abitstream. The bitstream may be transmitted to a decoding apparatusthrough a network or a storage medium.

The image/video information may include various types of informationaccording to an embodiment of the present disclosure.

In an embodiment, the alternative filter information may includeinformation on the number of alternative filters for the chromacomponent of the current block, absolute value information ofcoefficients included in the alternative filters for the chromacomponent of the current block, and sign information of coefficientsincluded in the alternative filters for the chroma component of thecurrent block.

In an embodiment, the alternative filter information may include indexinformation of the alternative filter for the chroma component of thecurrent block, and the index information of the alternative filter maybe based on truncated rice binarization.

In an embodiment, a maximum value (cMax) of the index information of thealternative filter may be equal to a value that is one less than thenumber of alternative filters for the chroma component of the currentblock, and the truncated rice binarization may be performed based on arice parameter (cRiceParam) value of 0.

In an embodiment, bins of a bin string for index information of thealternative filter may be entropy-encoded based on a context model. Thecontext model for the bins of the bin string with respect to the indexinformation of the alternative filter may be determined based on acontext index increment for the bins. In addition, the context indexincrement may be determined based on an index of a chroma component ofthe current block.

In an embodiment, the context index increment derived when the chromacomponent of the current block is cb may be different from the contextindex increment derived when the chroma component of the current blockis cr.

In an embodiment, the image information may include information on ALFclipping, and the information on ALF clipping may include a flagindicating whether clipping is applied to the chroma component of thecurrent block and clipping index information indicating a clippingvalue.

FIGS. 15 and 16 schematically show an example of an image/video decodingmethod and related components according to an embodiment of the presentdisclosure. The method illustrated in FIG. 15 may be performed by thedecoding apparatus illustrated in FIG. 3 . Specifically, for example,S1500 may be performed by the entropy decoder 310 of the decodingapparatus, S1510 may be performed by the adder 340 of the decodingapparatus, and S1520 to S1530 may be performed by the filter 350 of thedecoding apparatus. The method illustrated in FIG. 15 may include theembodiments described above in the present disclosure.

Referring to FIG. 15 , the decoding apparatus obtains image informationincluding adaptive loop filter (ALF) information and residualinformation from a bitstream (S1500). The image information may includevarious information according to the aforementioned embodiment(s) of thepresent disclosure. For example, the image information may include atleast a part of prediction-related information or residual-relatedinformation.

For example, the prediction-related information may include interprediction mode information or inter prediction type information. Forexample, the inter prediction mode information may include informationindicating at least a portion of various inter prediction modes. Forexample, various modes such as a merge mode, a skip mode, a motionvector prediction (MVP) mode, an affine mode, a subblock merge mode, ora merge with MVD (MMVD) mode may be used. In addition, decoder sidemotion vector refinement (DMVR) mode, adaptive motion vector resolution(AMVR) mode, bi-prediction with CU-level weight (BCW), or bi-directionaloptical flow (BDOF), etc. may be used in addition to or instead of anancillary mode. For example, the inter prediction type information mayinclude an inter_pred_idc syntax element. Alternatively, the interprediction type information may include information indicating any oneof L0 prediction, L1 prediction, and pair (bi) prediction.

In addition, the image information may include various informationaccording to an embodiment of the present disclosure. For example, theimage information may include ALF information, and the ALF informationmay include information described in at least one of Tables 1 to 42described above. For example, the ALF information may includeinformation on the ALF filter coefficients described above in thepresent disclosure, information on a fixed filter, information onclipping, and the like. Alternatively, for example, the ALF informationmay include alternative filter information for the chroma component ofthe current block.

The decoding apparatus generates reconstructed samples for a currentblock based on residual information (S1510). The decoding apparatus mayderive prediction samples of the current block based onprediction-related information included in image/video information. Thedecoding apparatus may derive residual samples based on residualinformation included in the image/video information. The decodingapparatus may generate reconstructed samples based on the predictionsamples and the residual samples. A reconstructed block and areconstructed picture may be derived based on the reconstructed samples.

The decoding apparatus derives filter coefficients for an ALF based onALF information (S1520). The decoding apparatus may derive filtercoefficients for the ALF. One filter may include a set of filtercoefficients. The filter or the filter coefficients may be derived basedon the ALF information. Alternatively, filter coefficients for a chromacomponent of a current block may be generated based on alternativefilter information.

The decoding apparatus generates modified reconstructed samples for thecurrent block based on the reconstructed samples and the ALF information(S1530). The decoding apparatus may derive filter coefficients for ALFfrom the ALF information, and generate the reconstructed samples andmodified reconstructed samples based on the filter coefficients.

For example, the decoding apparatus may generate modified reconstructedsamples by filtering reconstructed samples, and the filtering procedureof reconstructed samples may be performed using a filter based on filtercoefficients. The picture reconstructed by the decoding apparatus mayinclude reconstructed samples.

In an embodiment, the alternative filter information may includeinformation on the number of alternative filters for the chromacomponent of the current block, absolute value information ofcoefficients included in alternative filters for the chroma component ofthe current block, and sign information of coefficients included in thealternative filters for the chroma component of the current block.

In an embodiment, the alternative filter information may include indexinformation of the alternative filter for the chroma component of thecurrent block, and the index information of the alternative filter maybe based on truncated rice binarization.

In an embodiment, a maximum value (cMax) of the index information of thealternative filter may be equal to a value that is one less than thenumber of alternative filters for the chroma component of the currentblock, and the truncated rice binarization may be performed based on arice parameter (cRiceParam) value of 0.

In an embodiment, bins of a bin string for index information of thealternative filter may be entropy-encoded based on a context model. Thecontext model for the bins of the bin string with respect to the indexinformation of the alternative filter may be determined based on acontext index increment for the bins. In addition, the context indexincrement may be determined based on an index of a chroma component ofthe current block.

In an embodiment, the context index increment derived when the chromacomponent of the current block is cb may be different from the contextindex increment derived when the chroma component of the current blockis cr.

In an embodiment, the image information may include information on ALFclipping, and the information on ALF clipping may include a flagindicating whether clipping is applied to the chroma component of thecurrent block and clipping index information indicating a clippingvalue.

In the above-described embodiment, the methods are described based onthe flowchart having a series of steps or blocks. The present disclosureis not limited to the order of the above steps or blocks. Some steps orblocks may occur simultaneously or in a different order from other stepsor blocks as described above. Further, those skilled in the art willunderstand that the steps shown in the above flowchart are notexclusive, that further steps may be included, or that one or more stepsin the flowchart may be deleted without affecting the scope of thepresent disclosure.

The method according to the above-described embodiments of the presentdocument may be implemented in software form, and the encoding apparatusand/or decoding apparatus according to the present document is, forexample, may be included in the apparatus that performs the imageprocessing of a TV, a computer, a smart phone, a set-top box, a displaydevice, etc.

When the embodiments in the present document are implemented insoftware, the above-described method may be implemented as a module(process, function, etc.) that performs the above-described function. Amodule may be stored in a memory and executed by a processor. The memorymay be internal or external to the processor, and may be coupled to theprocessor by various well-known means. The processor may include anapplication-specific integrated circuit (ASIC), other chipsets, logiccircuits, and/or data processing devices. Memory may include read-onlymemory (ROM), random access memory (RAM), flash memory, memory cards,storage media, and/or other storage devices. That is, the embodimentsdescribed in the present document may be implemented and performed on aprocessor, a microprocessor, a controller, or a chip. For example, thefunctional units shown in each figure may be implemented and performedon a computer, a processor, a microprocessor, a controller, or a chip.In this case, information on instructions or an algorithm forimplementation may be stored in a digital storage medium.

In addition, the decoding apparatus and the encoding apparatus to whichthe present disclosure is applied may be included in a multimediabroadcasting transmission/reception apparatus, a mobile communicationterminal, a home cinema video apparatus, a digital cinema videoapparatus, a surveillance camera, a video chatting apparatus, areal-time communication apparatus such as video communication, a mobilestreaming apparatus, a storage medium, a camcorder, a VoD serviceproviding apparatus, an Over the top (OTT) video apparatus, an Internetstreaming service providing apparatus, a three-dimensional (3D) videoapparatus, a teleconference video apparatus, a transportation userequipment (i.e., vehicle user equipment, an airplane user equipment, aship user equipment, etc.) and a medical video apparatus and may be usedto process video signals and data signals. For example, the Over the top(OTT) video apparatus may include a game console, a blue-ray player, aninternet access TV, a home theater system, a smart phone, a tablet PC, aDigital Video Recorder (DVR), and the like.

Furthermore, the processing method to which the present document isapplied may be produced in the form of a program that is to be executedby a computer and may be stored in a computer-readable recording medium.Multimedia data having a data structure according to the presentdisclosure may also be stored in computer-readable recording media. Thecomputer-readable recording media include all types of storage devicesin which data readable by a computer system is stored. Thecomputer-readable recording media may include a BD, a Universal SerialBus (USB), ROM, PROM, EPROM, EEPROM, RAM, CD-ROM, a magnetic tape, afloppy disk, and an optical data storage device, for example.Furthermore, the computer-readable recording media includes mediaimplemented in the form of carrier waves (i.e., transmission through theInternet). In addition, a bitstream generated by the encoding method maybe stored in a computer-readable recording medium or may be transmittedover wired/wireless communication networks.

In addition, the embodiments of the present document may be implementedwith a computer program product according to program codes, and theprogram codes may be performed in a computer by the embodiments of thepresent document. The program codes may be stored on a carrier which isreadable by a computer.

FIG. 17 shows an example of a content streaming system to whichembodiments disclosed in the present document may be applied.

Referring to FIG. 17 , the content streaming system to which theembodiment(s) of the present document is applied may largely include anencoding server, a streaming server, a web server, a media storage, auser device, and a multimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. Into digitaldata to generate a bitstream and transmit the bitstream to the streamingserver. As another example, when the multimedia input devices such assmartphones, cameras, camcorders, etc. directly generate a bitstream,the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstreamgenerating method to which the embodiment(s) of the present disclosureis applied, and the streaming server may temporarily store the bitstreamin the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server deliversit to a streaming server, and the streaming server transmits multimediadata to the user. In this case, the content streaming system may includea separate control server. In this case, the control server serves tocontrol a command/response between devices in the content streamingsystem.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content is received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (ex. Smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like. Each server in the content streaming system maybe operated as a distributed server, in which case data received fromeach server may be distributed.

Each server in the content streaming system may be operated as adistributed server, and in this case, data received from each server maybe distributed and processed.

The claims described herein may be combined in various ways. Forexample, the technical features of the method claims of the presentdocument may be combined and implemented as an apparatus, and thetechnical features of the apparatus claims of the present document maybe combined and implemented as a method. In addition, the technicalfeatures of the method claim of the present document and the technicalfeatures of the apparatus claim may be combined to be implemented as anapparatus, and the technical features of the method claim of the presentdocument and the technical features of the apparatus claim may becombined and implemented as a method.

What is claimed is:
 1. An image decoding method performed by a decoding apparatus, the method comprising: obtaining image information including prediction related information, residual information and adaptive loop filter (ALF) information from a bitstream; deriving prediction samples of a current block based on the prediction related information; deriving residual samples of the current block based on the residual information; generating reconstructed samples of the current block based on the prediction samples and the residual samples; deriving filter coefficients for an ALF based on the ALF information; and performing an adaptive loop filter (ALF) process on the reconstructed samples to generated modified reconstructed samples of the current block based on the filter coefficients, wherein the image information includes index information of an alternative filter for a chroma component of the current block, wherein the ALF information includes alternative filter information for the chroma component of the current block including information on a number of alternative filters for the chroma component of the current block, wherein based on the information on the number of the alternative filters for the chroma component of the current block, the alternative filter information includes absolute value information of coefficients included in alternative filters for the chroma component of the current block and sign information of coefficients included in the alternative filters for the chroma component of the current block, wherein the index information of the alternative filter is based on truncated rice binarization, wherein bins of a bin string for the index information of the alternative filter are entropy-decoded based on a context model, wherein the context model for the bins of the bin string related to the index information of the alternative filter is determined based on a context index increment for the bins, and wherein the context index increment is determined based on the chroma component of the current block.
 2. The method of claim 1, wherein a maximum value (cMax) of the index information of the alternative filter is equal to a value that is one less than the number of the alternative filters for the chroma component of the current block, and wherein the truncated rice binarization is performed based on a value of a rice parameter (cRiceParam) value being
 0. 3. The method of claim 1, wherein the context index increment derived based on the chroma component of the current block being cb is different from the context index increment derived based on the chroma component of the current block being cr.
 4. The method of claim 1, wherein the image information includes information on ALF clipping, and wherein the information on the ALF clipping includes a flag related to whether clipping is applied to the chroma component of the current block and clipping index information related to a clipping value.
 5. An image encoding method performed by an encoding apparatus, the method comprising: deriving prediction samples of a current block; deriving residual samples of the current block based on the prediction samples; generating reconstructed samples based on the prediction samples and the residual samples generating residual information based on the residual samples; deriving filter coefficients for an adaptive loop filter (ALF) for the reconstructed samples; generating ALF information based on the filter coefficients; and encoding image information including the residual information and the ALF information, wherein the image information includes index information of an alternative filter for a chroma component of the current block, wherein the ALF information includes alternative filter information for the chroma component of the current block including information on a number of alternative filters for the chroma component of the current block, wherein based on the information on the number of the alternative filters for the chroma component of the current block, the alternative filter information includes absolute value information of coefficients included in alternative filters for the chroma component of the current block and sign information of coefficients included in the alternative filters for the chroma component of the current block, wherein the index information of the alternative filter is based on truncated rice binarization, wherein bins of a bin string for the index information of the alternative filter are entropy-encoded based on a context model, wherein the context model for the bins of the bin string related to the index information of the alternative filter is determined based on a context index increment for the bins, and wherein the context index increment is determined based on the chroma component of the current block.
 6. The method of claim 5, wherein a maximum value (cMax) of the index information of the alternative filter is equal to a value that is one less than the number of the alternative filters for the chroma component of the current block, and wherein the truncated rice binarization is performed based on a value of a rice parameter (cRiceParam) value being
 0. 7. The method of claim 5, wherein the context index increment derived based on the chroma component of the current block being cb is different from the context index increment derived based on the chroma component of the current block being cr.
 8. The method of claim 5, wherein the image information includes information on ALF clipping, and wherein the information on ALF clipping includes a flag related to whether clipping is applied to the chroma component to which an alternative filter of the current block is applied and clipping index information related to a clipping value.
 9. A non-transitory computer-readable storage medium storing a bitstream generated by a method, wherein the method comprises: \ deriving prediction samples of a current block; deriving residual samples of the current block based on the prediction samples; generating reconstructed samples based on the prediction samples and the residual samples generating residual information based on the residual samples; deriving filter coefficients for an adaptive loop filter (ALF) for the reconstructed samples; generating ALF information based on the filter coefficients; and encoding image information including the residual information and the ALF information, wherein the image information includes index information of an alternative filter for a chroma component of the current block, wherein the ALF information includes alternative filter information for the chroma component of the current block including information on a number of alternative filters for the chroma component of the current block, wherein based on the information on the number of the alternative filters for the chroma component of the current block, the alternative filter information includes absolute value information of coefficients included in alternative filters for the chroma component of the current block and sign information of coefficients included in the alternative filters for the chroma component of the current block, wherein the index information of the alternative filter is based on truncated rice binarization, wherein bins of a bin string for the index information of the alternative filter are entropy-encoded based on a context model, wherein the context model for the bins of the bin string related to the index information of the alternative filter is determined based on a context index increment for the bins, and wherein the context index increment is determined based on the chroma component of the current block. 